The /sound command handles sound requests
(same as having "accept sound requests" ticked). The sound-setting is handled with the /ebeeps command (same as having "enable sounds" ticked).
To the OP:
The button that currently shows "no sound" for each event has three states: 1) no sound 2) beep 3) <custom sound file>; thus a click on this button after the "beep"-state should open a popup where you can browse/select the associated sound file.
Having lost your sound settings, didn't you lose other settings as well? The sound event settings are stored in the mirc.ini file (section [waves]), together with virtually all your other mIRC settings.
Regarding the "roaming" folder: in Vista this is the default folder used by mIRC to store settings, and default subdirs for e.g. "downloads" and "sounds" will be created there as well. Are your "old" sound files located in another directory? If yes, is this directory a subdirectory of the folder containing the mirc.exe, and does the folder containing the mirc.exe contain a mirc.ini?
I'm asking these questions because: if mIRC finds a mirc.ini in the same folder as the mirc.exe, it will use this ini for it's settings, and the containing directory as "default" directory (and therefore look for sounds in it's "sounds" subdirectory). But if mIRC doesn't find a mirc.ini there, it will create a mirc.ini in "roaming" and take this dir as "default" (and therefore look for sounds in the "sound" subdir at roaming).
...hope I didn't cause too much confusion