JACK is a low-latency audio server, written primarily for Linux. It can
connect a number of different applications to an audio device, as well as
allow them to share audio among themselves.  Its clients can run in their
own processes (ie. as a normal application) or they can run within a JACK
server instance (i.e. as a "plugin").

jackd has to run with realtime priviledges. One way to do this on Slackware
would be to use set_rlimits. Since 12.2 there's another way. If you have
a filesystem that supports posix capabilities (reiserfs does not), you
can grant jackd the rights to run in realtime mode, even when started as
normal user with the following command:

  setcap cap_ipc_lock,cap_sys_nice=ep /usr/bin/jackd

If you use qjackctl to start jack, it will need the same capabilities set
to be able to start jack as non-root user. You can use the same command
just with 'qjackctl' instead of 'jackd'

jack optionally uses libsndfile, libffado and celt, which are all available
at SlackBuilds.org.