New figures also show a postcode lottery exists in parts of the country, with a lack of equal access to specialist therapists.
The study of a number of councils and health boards has been commissioned by the Scottish Parliament's health committee, which is currently considering a petition calling for the protection of quality speech and language therapy services.
The survey found services have been cut by 8.8% since 2011, with some health boards and councils cutting budgets by more than 20%.
Robert MacBean, policy and campaigns office for The National Autistic Society Scotland, said: "Speech and language therapy is one of a range of supports that can significantly benefit people with autism, and as such it should be accessible throughout Scotland.
"However, while some parts of the country have very good examples of support, this isn't the case everywhere. Many families are still facing a postcode lottery when trying to access basic services. The right support at the right time can make the difference between someone with autism experiencing isolation, loneliness and associated mental health problems or leading a fulfilling life as valuable members of their communities."
The therapists can provide help to children or adults affected by learning disabilities, or health conditions that affect speech such as stroke, dementia and autism.
North Ayrshire Council has announced it is cutting speech therapist numbers, but says it is investing in its own staff to improve the way it helps children with language problems.
Neighbouring South Ayrshire is phasing the funding out over the next two years.
Kim Hartley, Scotland officer for the Royal College of Speech & Language Therapists (RCSLT) said there were gaps in the data as not all councils and health boards had responded, but speech therapists had been concerned for a number of years that services were under severe strain.
She said: "Cuts on top of cuts do eventually have an impact on service access. We can see this in the increase in waiting lists across Scotland. We can expect lists to keep growing as demand grows,"
She added that Scottish Government policy and guidelines supporting speech and language therapy were failing to protect provision on the ground.
The RCSLT is calling for a comprehensive review of SLT provision and for the Scottish Government to respond to budget cuts. The body also says speech and language therapists and allied health professionals should have a stronger voice in local decisions on health and care services.
Trainee mechanic Andrew Caldwell, 18, of Kilmarnock, North Ayrshire, did not begin to speak until he was nearly five due to severe autism. He had the therapy from diagnosis at the age of three until he was in primary three.
His mother Claire said: "If it wasn't for speech and language therapy he wouldn't be able to talk still, wouldn't have gone to mainstream school and would not be attending college now."
A spokeswoman for North Ayrshire Council said it would be investing in the services to support its objective of focusing on early intervention and prevention, but added that after a full review it has now developed "a more sustainable model to provide SLT support to children and young people." She added: "This new approach will see the council funding a reduced number of speech therapists."
A South Ayrshire Council spokeswoman confirmed a decision had been taken to phase out funding following consideration of alternative provision available within local communities.