Your post said you didn’t want to have “great expense” but you didn’t specify a budget. So that’s why I mentioned the solution we use, which does require a $50 echo Dot (often on sale for $35). It’s a quick and pretty easy solution.
But if you really don’t want to buy anything extra, the issue you’ll run into is that playing music usually requires both the ability to set up a rule in smartthings, and a speaker device which has “play music” options. Different speaker devices have different options in this regard. Sonos, Echo, and IKEA Symfonisk all have some options, but somewhat different ones.
And there’s no easy way that I know of to have the official smartthings features play music on your android phone, unless you use Bixby routines on a galaxy phone, and you didn’t say you had a galaxy phone.
So then you have to start looking at third-party apps, which is why I mentioned the combination of Sharptools and Tasker. Both of those are paid license apps, so they may be outside of your budget as well.
If you have a Samsung galaxy phone, you can probably do this with a Bixby routine without needing anything extra. The music will play on your phone.
Otherwise, the easiest way to do this is to get a speaker device which has play music capabilities that smartthings can access.
Alexa dot is probably the least expensive simple option, often on sale for $35, and then you don’t need to buy anything else.
IKEA Symfonisk is a $100 device with better sound and more direct integration with the smartthings app, so for people who want a better quality speaker, it’s a popular choice.
And as you mentioned, Sonos has a direct integration and lots of options, but is very expensive. I think the least expensive model is about $350.
If you don’t want to buy any speaker device and just have the music play on your phone, then there isn’t anything built into the official smartthings features that will do this for non-galaxy phones. There are still ways to do it, but it sounds like they would feel too complicated for what you want.