Hello,
Testing the 5.0 all-in-one from squeak.org in a Mint 17.2 running in VirtualBox (Windows 8.1 host), I see that
SimpleMIDIPort midiIsSupported
returns false with the linux VM, and true with the Windows VM (run though Wine).
So it seems to me that the linux MIDIPlugin is broken (or am I missing something ?)
Stef