Hmm, maybe it's an issue in seatbelt itself?  This

  $ sandbox-exec -p '(version 1) (allow default) (debug deny) (deny file-write* (regex "foo"))' touch foo

adds no entries to system.log.

Indeed we sent Apple a radar, we think seatbelt logging is broken in the upstream OS.

Right. Thanks for confirming.

In the meantime, do you have any hints as to how to go about figuring out what to allow in order to migrate from AudioToolbox warm-up to explicit sandbox directives?

No, we unfortunately there is not a great way to debug at the moment. You can use dtruss to track failed syscalls, but that won't handle things like Mach lookups.

I encourage you to file an additional radar to get more attention on the issue. Ours is rdar://41644488.

Good idea.  Filed rdar://43271636.

I've noticed the problem in macOS 10.13, but it still works in 10.10 (and perhaps some other versions too).  Unfortunately, the right sandbox directives also sometimes depend on the OS version.

Yes, I tracked down the regression to starting in 10.13.4, and it also affects the 10.14 betas.

Fixed in 10.14.1 18B57c.