PAC Files, as has been figured, are:
a) A format re-used from the PSO games
b) basically a bunch of PCM wave files, little endian, 16 bit, 22,050 Hz, mono, chained one after another with a bunch of headers to tell the game which sound is which.
The Heroes PAC format is a tad bit different, but nevertheless, the written-for-PSO tool PAC Manager version 0.4 can handle these files relatively well. Previous versions and the following version 0.5 cannot handle the Heroes PACs for various reasons, mostly since those versions are almost exclusively geared towards the actual PSO .pacs, complete with reference-addresses built-in that won't do no good with Heroes' files.
PAC Manager will give you a file list, labelled Sound 01-XX, depending on file-count. The first 10 files (at least in Bank0.pac) are Header data not usually found in PSO PACs. PAC Manager 0.4 erroneously reads these as 52-byte-long sound files each. Skipping past those, we got the actual sounds.
Using BlazeHedgehog's guide on how to listen to the sounds in the pack is a good way of figuring out which sound number is which.
It's a good idea to extract the sound you want to replace first.
The file you want to insert should be no longer than the original sound, as PAC Manager cannot expand the PAC-files. Hence it's a good idea to have the original out for size-comparison's sake.
The file you want to import must be a 22050 Hz 16-bit Mono PCM Wave-file.
If all went well, your sound is now in the .pac file. Remember to save the modified .pac, though.
Finally, you must do one change to the archive. Open and compare the .pac header of the modified file with the back-up file in a hex-editor and you'll see that there's stuff missing. This is likely due to, as established, PAC Manager assuming the Heroes .pac-header being 10 sound files instead of the file's header. This can be fixed by simply copying and pasting the first 352 bytes from the back-up into the modified file. Saving that, the sounds should now work flawlessly in the game.
If I explained anything in a dumbass, not-understandable way, do hit me with a heavy object and correct me.