The only way audio from your location gets
to a NINJAM server is via the NINJAM client, yes
.
So if you mute the channel
prior to the client, you should hear nothing from your own set up (because you only hear yourself through NINJAM). You should still hear anyone else playing but they should also NOT hear you.
If, in this case, you still hear yourself, you need to check your signal path to make sure it's only passing through NINJAM. If the others can hear you, you've something strange happening and need to check the routing to the client.
Next (once the above are working), if you mute yourself
within the client, you're still being sent to the server and out to others. So
you shouldn't hear your sound (because you only hear yourself through NINJAM) -- but others should hear you.
If in that second test, you can hear yourself, you've a second input source somewhere (maybe you, maybe elsewhere) - but that doesn't immediately sound like the problem here.