Here to help jay, soldering to the speakers or motherboard should not affect sound quality. New modders that solder to the speaker contacts for the first time get frustrated because when you heat them up they almost always fall off, and you wind up having to put A bit of solder where the contacts used to be so the speakers connect. If you solder the wire to the speaker, this eliminates that problem, but It can be harder to wire to the left speaker as it is incased in black plastic.
And for your second question, depending on what type of psp you have will probably determine whether or not you can keep your umd. You will just have to make sure that the speaker does not enter the umd bay area too much, or your umd will not spin right. It's possible to do it, but the psp is going to be pretty thick.
Good luck!