They commited suicide.
It's plain as day: listen to the tape and treat Voice 1 like the reluctant guy talking them all out of a suicide pact. It makes perfect sense that way.
I'm sure some of you know this, and sure enough when re-checking the wiki, someone figured this out too and added into Vault 11's article. Hell, I'm sure there are others reading this wondering "yeah, duh. You're point?" I know I know, having realized that now, I feel SOOOO stupid for having taken a good 2 and a half years to figure that out. Nevertheless, for the life of me I can't recall a single post on THESE forums saying that, though I recall several talking about the mystery of who shot the other four, so I thought I should bring it up.
But yeah, suicide. Voice 1 is a pansy and doesn't want to, all the other voices feel so guilt-ridden they commit suicide. The whimper at the end isn't a final breath of someone who's been shot, but a sigh by someone unable to talk his only human contacts out of suicide.
I dumb. :c