You can mute the corresponding tracks in pattern view or master view. Synth is a sound source like a sample. You also cant mute individual instruments.
It still seems inconsistent to me because having separate inputs for the synths on the master mixer view means they are not like other instruments (which are part of the dry mix). They behave more like external midi synths that feed back their audio via separate “line ins” so why not also have a mute for those inputs.
@retro_frog I already filed a bug report, look at the post above. Agree with inconsistency. Audio as signal and midi/synth as velocity should give a consistent visual feedback to the user.
Regarding mute buttons: then you actually SEE the signal on the corresponding tracks where you also can mute them.