try {
      Synthesizer syn = MidiSystem.getSynthesizer();
    } catch (MidiUnavailableException e) {
      e.printStackTrace();
      System.exit(1);
    }
    try {
      syn.open();
    } catch (MidiUnavailableException e) {
      e.printStackTrace();
      System.exit(1);
    }

When logged in as a normal user fails with
javax.sound.midi.MidiUnavailableException: Can not open line
        at com.sun.media.sound.SoftSynthesizer.open(SoftSynthesizer.java:1132)
        at com.sun.media.sound.SoftSynthesizer.open(SoftSynthesizer.java:1036)
        at music.Synth.setup(Synth.java:42)
        at music.Synth.main(Synth.java:171)
Caused by: java.lang.IllegalArgumentException: No line matching interface SourceDataLine supporting format PCM_SIGNED 44100.0 Hz, 16 bit, stereo, 4 bytes/frame, little-endian is supported.
        at javax.sound.sampled.AudioSystem.getLine(AudioSystem.java:479)
        at javax.sound.sampled.AudioSystem.getSourceDataLine(AudioSystem.java:606)
        at com.sun.media.sound.SoftSynthesizer.open(SoftSynthesizer.java:1066)
        ... 3 more

As is quite common with the PI, it works fine when logged in as root.

Regards
Don