Thanks for jumping in here with that fix. I hadn't seen this thread until today, but you beat me to the punch.
The problem is that 10.9 cannot support HAP playback on AV Foundation. While we no longer officially support anything less than 10.10 (and that will become strict in September 2018 with the release of Isadora v3) I discovered the problem and fixed it for @artoo. Hopefully it will eliminate the issue for @DanielB.
Let me say it again: Isadora v3 will not run on anything earlier than 10.10. Please keep that in mind as we head towards that release.