Maxima is a software application that allows users to interactively mix audio tracks using a curses-based interface or from the command-line.
One unique feature that stood out to me was the numeric level readout in addition to the sliders, making it easier to track levels. Another plus is that it should work on any terminal with a screen size of at least 80x24.
The latest version, 1.4, includes some minor fixes such as corrected slight misalignment of usage lines in rexima.c (usage_help) and an addition of an old getopt() replacement to avoid issues with glibc's treatment of level decrements as unrecognized options (thanks to Alexandra Walford for spotting the bug).
Overall, I would highly recommend Rexamia for anyone in need of a reliable and straightforward mixer for their command-line or terminal-based audio production needs.
Version 1.4: N/A