Hi Rob, thanks for replying.
I’ve read all the links in your reply and I’m still stumped. 
I’m not loading a sound file but playing a series of tones that I create using code, but I don’t think that should be an important difference.
I created a simple series of visual events in Builder - text that flashes in the middle of the screen on each trial - and I’ve put code in the ‘begin routine’ tab to start the sound loop. But all I get is the sound loop being played once, and then the visual trials running. Would you mind taking a look? It might be something really trivial that I’m missing.
You can ignore the code called make_tones, it’s more complicated than it needs to be for this example but it makes a small series of tones that can then be called with my_tones().
[background_tones_example.psyexp (17.8 KB)]
I’ve written a more elaborate post about my issue here.