The volume and quality on the Desire S ringer and mp3 player/radio are average, although better than on previous HTC phones. If you're in Europe, volumes are usually capped to comply with EU rules. Some mp3-player companies don't seem to comply or they may allow the user to change between European and Rest of the World settings. I haven't found anything conclusive one way or the other regarding HTC and their position on this.
There are review sites which test the decibel level of phones - voice, music and ringer - e.g. here
. However, given that there maybe at least two models of some phones (a European and Rest of the World version), you need to be clear which model they were testing. E.g. was it a European model or Rest of the World model, as this might mean the phone will be volume-capped? Again, the picture here is unclear.
Then there's the volume vs quality debate. Some phones may be louder, but their sound quality won't be great.
Swings and roundabouts. You pays your money...
PS Have you tried any of the 'volume boost' apps on Android Market? (But see above re: volume vs quality.)
And a good pair of headphones/earbuds always help.