Try to install alsamixer or pavucontrol. Or try the new Linux-libre versions (by default Trisquel 6 use Linux-libre 3.2 - LTS), currently the version is 3.8:
$ sudo add-apt-repository ppa:linux-libre/ppa $ sudo apt-get update $ sudo apt-get install linux-libre $ sudo apt-get dist-upgrade https://launchpad.net/~linux-libre/+archive/ppa http://jxself.org/linux-libre/
