How to mute or disable laptop speakers when listening through bluetooth earphones ?

Hallo, forum

What it says in the title, really. Trisquel supports bluetooth on my Lenovo ThinkPad T400, and I can listen through in-ear devices. It would greatly please the rest of the household if I could mute the sound coming through the built-in speakers. I have tried the obvious, plugging a jack into the headphone socket, but that mutes all audio, and I get nothing through bluetooth either.

Is there a simple fix, please, either via the Trisquel GUI or the Terminal ?

Cheers

Ian Graham
Wales UK