If we're saying forget scale...

For less than $50 you can get a Bluetooth rechargeable speaker at any electronics or even grocery/drug stores, Walmart, Target, etc. That way you get loud sound controlled by your smartphone as long as you stay in decent range.

I remember buying a mini cylindrical one by SOS or something for like $25 at Target as a gift for someone. It could easily fit on a 1/10 or larger vehicle and it sounded pretty decent for the price.