Here is the solution: amixer (or alsamixer, which has the ncurses interface).
To use alsamixer is easier due to the straightforward interface. If you get the following error message when you type ``alsamixer'' in the command
alsamixer: function snd_ctl_open failed for default: Connection refusedthen do the following steps:
$ rm ~/.asoundrc and then dowhere the foo is one of the sound cards listed by `asoundconf list.'
$ asoundconf list
$ asoundconf set-default-card foo