I have made several custom firing sounds for my weapons. However, if I am firing one weapon and someone else is firing at the same time I only hear my weapon instead of playing both sounds at the same time. Why is this and how do I fix it?

What's the sound type set as, at the top of the sound tag?

They are set to weapon_fire

What are the other sound settings? (Specifically: compression, frequency and mono/stereo).

Halo CE does not like it when impulse sounds (anything that's not music or cinematic pretty much) are at 44khz instead of 22, and I also think it's not so happy when they're compressed with something other than Xbox ADPCM. I don't think mono/stereo is as important but if it comes down to that I would also make your sound mono.

Frequency is 22kHz, compression is ogg, and stereo encoding. I think the problem is with the compression. How do I compress it with Xbox ADPCM?

You need the Xbox ADPCM codec on your computer (though if you're running Windows above XP this is nigh on impossible). You really just need to save the sound as you normally would - all that changes is you compile with "xbox" instead of "ogg" as the compression mode.

ex: tool sounds sound\sfx\noise xbox 1 instead of tool sounds sound\sfx\noise ogg 1

Before you try that though, see what happens when A) you compile as normal wav (same as above, just enter wav instead of ogg or xbox) and/or B) you make it mono not stereo.

I have Windows 7 64 Bit, so I can't do it?

It might work for you, I don't know. It doesn't for me. You can get it (Google usually turns it up) and try for yourself. But like I said, also see if the problem persists when you compile using other settings.

If not, you can always use Virtual PC to make a 86x windows installation on. I found this extremely useful for Halo Sound operations. I have a Windows XP Virtual PC and the Sound Recorder can re compress WAV's to correct Halo Format.

All he needs to do is to make the sound mono, not stereo. It does not matter if it is xbox or ogg; in fact, OGG files work better.