Hi Erik,
According to the FSF, the MIT License (also known as
the X11 license)
is GPL compatible. See:
http://www.gnu.org/philosophy/license-list.html#GPLCompatibleLicenses
thanks for the info and the pointer which I could have found by
myself :) . Glad to see that I can use the classes.
BTW: On the RtMidi homepage there are also ready to use audio classes.
See
http://www.music.mcgill.ca/~gary/rtaudio/
for details.
Best regards
ce