Are there any software TNC's (like SoundModem) for Linux?
This is limited to the ones which deal with BPSK1200 (used by cubesat projects).
For some cases Warbler will suffice.
Soundmodem (the original from Thomas Sailor) is still (Debian Jessie) available for Linux. I used it recently with aprx to create a software TNC with a generic USB sound card for APRS purposes.
apt-get install soundmodem