I was wondering about this issue as well. My OG Mini had considerably lower speaker volume than the iPad 3 I had switched from. I have the Air now and the built-in speakers are the loudest I've heard from any iPad to date. Hopefully the lower volume is just a manufacturing defect that they will eventually correct on the rMini.
When my 1st gen mini updated to iOS 7, I definitely noticed a volume drop. The rMini sounds exactly the same. The quality of the speakers is actually very good, but I wish there was more volume available.
My original Mini I could have down to the lowest volume setting and hear my games fine. On the rMini I've noticed I have to have the volume turned up to at least 4 bars for the same level of volume. So I do think the rMini has a lower volume than the original Mini. I can't comment on other iPad hardware, however.