video and sound percentage makes the processor's core works slower, so it can make the game works faster...or slower. Let's see, sometimes a game don't use the cpu at his max power, so if you make a cpu core work at fullspeed, you are wasting processing time. You know that the Dreamcast has limited resources, so you can't overhead the CPU with this things, maybe you can in your desktop PC. That's why this options are there, just play with them, some games can work better, others can be slower, others can crash...
About the development of MAME4ALL, well, i think that CHUI is concentrated with another project (allegro maybe?), but i know that there's a MAME4ALL 1.0 for GP2X, based in a newer MAME (the lastest dc version is based in 0.34 with some new games added).
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