ah. the dreaded auto-connection feature of mplayer et al. :)
if you don't want anything else (auto-)connected to system/playback_1 and 2 ports but jack_mixer/MAIN L and R, please add a patchbay rule for exactly that, setting the exclusive flag on system (client) input socket side:
jack_mixer/MAIN L, R -----> system/playback_0, 1 (exclusive)
ah. the dreaded auto-connection feature of mplayer et al. :)
if you don't want anything else (auto-)connected to system/playback_1 and 2 ports but jack_mixer/MAIN L and R, please add a patchbay rule for exactly that, setting the exclusive flag on system (client) input socket side:
cheers