First of all, if this is in the wrong forum/not allowed, please do what you must with this topic.

It seems the US release lacks an option to change the annoying US vocals to the Japanese ones in game. So here's a quick fix for people with modded consoles. I did this on Xbox of course, and filenames can easily be changed back and forth with the remote FTP login on Xbox. I'm assuming this same technique can be done on PS2/GC copies simply be renaming the files before burning or transferring or whatever.

Anyway, in the /DVD directory is:
SH_VOICE_E.afs
SH_VOICE_Jafs
These are the main vocal packs. In case it wasn't obvious, the naming scheme for Heroes is J= Japanese, E=English. So, simply switch the filenames (make E->J and J->E). Also in /DVD is:
SNG_TBSFA4.adx
SNG_TBSFA4E.adx
SNG_TBSFA4J.adx

This is Team Chaotix's Team Blast sound effect, in which their voices are used. SNG_TBSFA4.adx seems to be the default (but it doesn't look like it's used), while E and J are the English and Japanese ones (yes, this means that English is on there twice). Simply switch around the E and J to fix that. In /DVD/sound is:
bank3e.pac
bank3j.pac
I don't know what these do, but guessing from the E and J, I'm assuming there are some differences. Once again, simply switch the E and J.

This should effectively switch over your audio to Japanese.

This doesn't change the CG movie audio though. There also seem to be some minor timing issues with the subtitles, but this is probably because Engling/Japanese lines are longer/shorter.

For those of you wondering, in /DVD/movies:
S1_Jp_XD.sfd
S1_Eng_XD.sfd
is the opening video. The only difference is the title logo at the end.

Yeah, I did that too, goodbye to the horrible english voices!

I don't know why do the Xbox games force you to change the game language via the Dashboard, the PS2 version let's you choose any voice+subtitle combination from the options screen (but looks ugly and has worse framerate), the GC version simply can't, because there's not enough space in those small discs to hold both languages.

I just hope there is a code that unlocks the voice+sub selection screen, It would be a bit more fair, instead of (ugh) hacking the poor game.