RoJeC wrote:increase latency (buffer size) in Preferences > Audio tab.
Already tried that, and I don't think that would've applied to playback through the internal audio would it? Anyway, I found the solution. I was going nuts (have a gig in the morning!), went through every single setting (even the ones that didn't seem to apply) and found that DVS control was enabled for Deck 1. I don't know why this should make a difference, but it did. Disabling it seems to have solved the problem. I use my CDJs in HID mode so this setting doesn't affect me.
I am going to suggest that this is a bug - enabling DVS control on one deck shouldn't affect audio playback if no DVS signal is present, should it?
Anyway, thanks for the quick response.