Hello- my son is 3 1/2 years old. He has a signifacant speech delay and seems to tune people out. Are these signs of autism?

Get hearing tested. Before considering autism make sure you have his hearing tested.
Possibly. They are non-specific signs of several possible things. Start with his pediatrician and a hearing and speech and language evaluation now early intervention for this is optimal.
All good answers.. "speech" = motor aspect of pronunciation. "language" = both comprehension of spoken words (receptive) & verbalization of thoughts (expressive). Call your local school district's child find team to assess all areas of your son's development & evaluate for autism to determine his eligibility for early childhood special education. Firstsigns. Org has screening tests for both. Please don't delay.
Maybe. Both are features but kids who struggle with communication may try to avoid situations where people are likely to try to engage them in conversation. I would have the kids doc do an autism screening or you can find some online & fill them out. Then proceed to your pcp and find how to get local help.