When Sven is playing at the inn, every song interrupts after a single animation, at least for me. Then he starts on a new one. Studying the behaviour more closely, it seems like he is trying to take a step towards an AI marker, but is immediately teleported back to the bard spot. Now looking at his AI package, SvenPlayInn10x16 (0001CB1B), I noticed it is set to Sandbox. My guess is therefore that the interruption is caused by him trying to sandbox around the bard spot, but that behaviour is immediately interrupted by the bard quest and so on.
This is not ideal. As an example, Luaffyn's corresponding package, WindhelmCandlehearthBardPackage (00047CB0), is based on the UseIdleMarker template, which is not a sandboxing package. Her songs do not interrupt. Therefore, I expect using a similar treatment for Sven would resolve the bug.