Before we dig in to the sounds of the week, there are a couple of notes I'd like to mention; items to consider before directly targeting speech sound production with children exhibiting significant speech disorders.
Consideration #1: Functional Communication
My first order of business with every child is to ensure that they have a functional means of communicating. While speech (verbal expression) is something we all want for all kids, the ultimate goal is that they can effectively and efficiently express their wants, needs, thoughts, feelings and stories to any communication partner. For many kids, speech will come. For some it does not. 
When appropriate, I do work on speech sound production skills - but not at the expense of a child's developing language skills and/or their ability to functionally communicate. Before I start directly targeting speech production skills, I ensure there is a means for the child to communicate (ideally in all places, with all people). How this looks varies from child to child based on their strengths, weaknesses, family culture, and a host of other factors. Let me give you three case examples.
Client #1 - let's call him Jonny. I first started seeing Jonny shortly after his 2nd birthday, ~3.5 years ago. At that time he had no diagnosis. He was completely unresponsive, had no interaction or play skills, no functional communication (speech or otherwise), and extreme behaviors (such as head banging and property destruction). A few months after my initial evaluation he had been diagnosed with Autism Spectrum Disorder and Sensory Processing Disorder. At that time he had no interest in or engagement with people. He was not ready to work on speech production skills.
- The first year of therapy focused on developing social interaction and engagement. Trying to teach him to say a sound, or exchange a picture, or produce a sign at this stage in his development would have been fruitless. He had no interest in engaging with a communication partner and did not have an understanding that other people could be agents to make good things happen. I spent most of my time with him on the floor imitating him, setting up interaction routines, and encouraging his tolerance of my intrusion and presence in his world.
- Midway into our second year, Jonny was seeking out (expecting.. demanding!) attention and interaction from others. He was ready to really focus on establishing a system of communication. By this time he had developed use of basic PECS and some signs for requesting.
- At the end of our second year working together, he was vocalizing a TON and trying so hard to express all the thoughts in his head - he clearly had a lot to say! But he was getting so frustrated by communication partners not understanding him. By this time, he had officially been diagnosed with Childhood Apraxia of Speech (CAS) in addition to his other diagnoses. It was really time, and his family was ready, to focus on implementing a comprehensive augmentative-alternative communication (AAC) system. For him: an iPad Mini with the TouchChat HD - AAC with WordPower communication app. Functional communication, check! Frustration started to ease. And Jonny started making real attempts at more than just requesting. Comments! Directives! Narratives! Relating information! Asking questions!
- About 2.5 years into therapy, we were ready to have the main focus of therapy include speech production skills. He could now focus, tolerate the hard work, and had good turn-taking and concept of imitation. Jonny is one of the core clients around whom I have been designing the sound of the week project. AAC continues to be an ever-present tool. However, about a year into the focus switch, his speech is really blooming! But we needed all of that "back work" to get him to a place of wanting to communicate, to (effectively) persist in his communication attempts, and to have the skills (and tolerance) to work on speech production.
Client #2 - let's call him Mike. I started seeing Mike about a year ago. He was 7.5 years old and had a diagnosis of Autism Spectrum Disorder. Parent focus was strictly on improving speech intelligibility. Mike was minimally verbal. He had some learned words and scripted phrases to make requests. And he was highly imitative, attempting to repeat any word I said. But he was functionally non-verbal. He rarely initiated communication and had little interest in communication partners. Upon examination, it was clear he had a motor speech disorder. His jaw and lips were so tightly clenched that he was unable to accurately produce any speech sounds.
- The first several weeks of therapy focused on social interaction and engagement, implementing a functional communication system (in this case, only used within therapy due to family culture) and establishing an expectation of communication. Nothing happened without an attempt at communication. I waited him out (and held out on desired items) frequently. I modeled language and communication continually.
- A couple of months into therapy I started to implement the "sound of the week" with Mike, with tweaks to individualize it to his needs and pacing.
- Now a year into therapy, Mike is spontaneously commenting, directing, and using novel and more complete utterances to communicate. He is much more engaged. Again, we needed the "back work" to first get him motivated to communicate and engage.
Client #3 - let's call her Susie. I started working with Susie this past fall. She is 3 years old with no diagnosis other than the motor speech disorder I observed at the time of her evaluation. She came to me for poor intelligibility which turned out to be due to poor control of her jaw, facial muscles and tongue. She has a large vocabulary, and is usually able to make her wants, needs, thoughts, and feelings known, although with some struggle for both her and her communication partner. Once she warms up, she eagerly engages and interacts.
- Susie was ready to jump right in and start working on speech production skills. She is always eager to actively engage with me and spontaneously communicates all the time. No pre-speech or communication skills needed to be addressed first.
As you can see from just these three case examples, the path, timing, and needs vary greatly from child to child. Some may be ready to jump right in and start working on specific sounds or sound sequences. Others may have different skills that need to be addressed first. Before you can work on speech production, the child must be interactive, take turns, and exhibit imitative behaviors. For some children that may mean that your first goals are establishing joint attention and joint interaction (like with Jonny), and developing turn taking and imitation behaviors. Learning to speak is all about imitating what we hear others say. And learning to communicate is all about taking turns within dialogues or communication exchanges.
Please, please, please! Do not sacrifice a child's language development and ability to communicate at the altar of speech production.