I know this was an old post, but I've been thinkng about sound, too.
First of all, I doubt you could alter what's in a MR saber, since most likely all the sound is hard-wired into a sound chip.
Also, I'd be surprised if converting to a Lux made the sound louder. In fact, when my batteries start dying, the blade seems to fade before the sound, so the speaker is getting plenty of power.
On top of that, the #1 factor affecting the sound quality on the MR is the fact that it sounds like 8-bit sound at a low sampling rate. During the idle hum, you can clearly hear "static" in the background, which to me sounds like the aliasing of low-res audio. If MR upgrades to a better chip (which probably means going from a 64k memory chip to somelike like 256k), the sound would be MUCH clearer.
Memory is so cheap now, I'm sure MR could do this sort of upgrade without affecting price. Now that we've heard they are going to include movie-accurate sounds on the sabers, this means new sound chips - hopefully it will be upgraded sound quality as well.