It would be helpful to understand why it wouldn’t be functional at this stage to teach the child how to use verbal communication? Surely speech is functional and supports the development of expressive language and increased opportunities and access to social interactions so those skills can also develop? From my experience AAC doesn’t provide the same access to social interactions or expressive language in the way that speech does.
As a parent as well it is wonderful to hear the sound of your child’s voice and this would always be a preference I am sure.
I think learning speech is ideal as it is the most efficient form of communication. With a growth mindset every child is capable of learning these new skills. AAC is a great tool and can be used alongside the PROMPT approach to aid understanding etc. I don’t teach or recommend AAC as a primary communication tool but do use it to aid the development of concentration, following routines, making choices etc.
I suspect this little girl should be able to learn how to use her vocalisations and connect meaning to her vocalisations when taught correctly and systematically. I am sure with PROMPT, she will have a very good chance of learning new motor speech skills.
I worked with a 3 yr old non-verbal, downs child and despite several years of SALT, still did not speak, using only makaton and AAC to communicate. With PROMPT the child was able to connect meaning to vocalisations and communicate in 2 word phrases after 4 months of therapy.
It was so rewarding to experience the power of PROMPT in this way. AAC was still widely used to aid her learning etc.
Don’t give up on teaching her how to speak and I’m sure your efforts will be well worth it.