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.
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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