Ivar has been tested on a Mac II, Mac IIx and Mac IIfx running System 7.0 w/o TuneUp. It appears to be quite stable.
Note that on a loaded network the list of sounds can take a while to be displayed. If it doesn't appear at all, the other machine is probably running some program which doesn't give any time to background tasks. (Like a game or a badly written screensaver)