There's not any way to fix the sound in ZSNES. You would have to re-port the entire emulator (not a trivial task) from another core that had more accurate sound for those games. However, you will find if you did that, you would break lots of other games. Trust me, the core in ZsnexBox is the best core there is. The latest PC version is not the best. With every "new" version of ZSNES over the last 3 years, it seems it gets more crippled overall. They might have fixed the sound issues in some games however, it was at the expense of breaking other things. See how far you get in Super Mario RPG in ZsnexBox 1.3 (the last version that used a newer core)
There was a LOT of research in finding the core that ZsnexBox uses. It is the best overall core there is. There are a few games that have minor sound issues, but this is not some major thing that affects all games. Honestly, the only game that I know of that has the "metallic" sound is Street Fighter 2 Turbo. I've never noticed anything in the Megaman X games. As far as sound issues go, the sound core is far more accurate overall than the one in XSNES9X (try playing any of the square games. The sound is nowhere close).
This is all a moot point anyway. That's because virtually nothing runs fullspeed in any SNES emulator on the Xbox other than in ZsnexBox. Most games will average 45 FPS (or lower). Someone could port BSNES and get perfect emulation. The reason no one does is because the Xbox couldn't run the games fullspeed. Who wants to play games with frameskipping? No one. That's the same problem with XSNES9X or Snes9xBox. Neither one can run games fullspeed. Zsnexbox can run virtually all games fullspeed, with very accurate sound overall, and has 10 times as many features than any other SNES emulator on the Xbox. It's the best there is. There are a few games that do not work correctly on ZsnexBox (Starfox 2). They should be played on another emulator.
Overall, anyone not using ZsnexBox for 99% of their SNES emulation is only cheating themselves. Until a system (like the 360) gets a port of BSNES, then ZSNES is the way to go.