@user1464279374
First time answering on here; prefacing that while my brother is on the spectrum and I have >10yrs working with/teaching children and young people with autism, I am NOT a parent myself. I'm aware the experience at home is very different, and having never had to implement this under my own roof, please feel free to ignore me as appropriate. :)
A lot of the younger children I've worked with have a combination of sensory issues (like Lottie mentions with her son) - sometimes ear defenders help to reduce this, but overwhelming visuals (bright colours) or too many people can play a part as well. If that sounds like a description of the music class, it may be kinder to look at alternatives.
What can help, if the behavioiur is due to anxiety, especially for children who are relatively non-verbal or struggling to communicate, is to use either a visual timetable or a 'Now' and 'Next' board. A 'Now' and 'Next' board, or First-and-Then board, basically has two boxes that you put simple pictures or words on showing your current activity and what is coming afterwards. E.g. now 'lunch', next 'play'; now 'music' next 'home'. It helps children feel secure and that they know what is happening. The only rule is you have to stick to the board - if you've put play next, you have to follow through so your child can trust the board.
It might be that he really doesn't like the class/is having sensory trouble that makes it hard to access, but helping him know what's coming up next may help.
He'll likely settle back into SALT if he was making progress before; breaks are really difficult for people with autism to take and come back from, but the neural pathways he's been working on in SALT haven't vanished - he's probably not quite as 'at square one' as he seems. If you're going to keep going with any classes, I'd prioritise SALT, since communcation's where you think his frustration lies.
If he continues to be distressed at the music group classes, maybe try meeting one of his friends (if he's not close to any of them, choose a parent-child combination you feel comfortable with) at a familiar playground or similar during quiet hours? It gives him the option for social interaction, and you can agree on a set time so he knows when he is going home (I know play-dates are often at each other's houses, but it can be super stressful if another child is in your house touching your things and you can't tell them not to).
It's true that some children find nursery and school distressing, but for others the structure and consistency can be really beneficial. There are set rules about what happens when and what is and isn't okay, and these boundaries can be comforting for children with autism. I knew one little boy who at 4yo was an adorable little hell-raiser (because he was constantly anxious), but a year later, having started school, was used to the idea that days had structure, used a now-next board regularly and so could be calm enough to engage with others non-aggressively. His was an extreme change, but it does happen. I think the main precaution, whatever you decide, is to ensure the nursery or school he eventually attends have worked with children with autism and are knowledgeable on how to support him.
Again, I know it's easy to say this when it's not my child. What I can say is you're doing an amazing job - for him to be diagnosed (getting a diagnosis can be a huge challenge in itself) and attending classes so early is in itself a credit to you and your efforts to support him the best you can.
TLDR/Summary: Try a visual timetable/now + next cards; try ear defenders if he'll put them on; hang in there and be patient with yourself as well as your son - you're doing your best, and that is enough, even if it doesn't always feel like it.