Just to way in here, i really don't think you can do much more, you can maybe make it louder, but i have to admit that the "onboard" sound in the N900 IS the best i have heard of all phones/handhelds.

If you wanna do something else then look into modding it and putting the 2x3W AMP between speakers and JACK out, powered from the battery and someway off turning it all on when needed.

You can use other amps then the VMA2012 i showcase in my DIY audio thread (in signature), there are loads of them, and some are much smaller (E.G. more flat)

From what i remember from last i had my N900 taken apart & last i had one of the N95 taken apart is that the speakers are very similar, but the N900's are better and the cabinets in the N900 are better tuned, if you take a N95 and compare with a N900 the N900 wins in quality, the N95 may be a bit louder but i can't see the point when it begins to sound like sh*t.