While Sven the bard is playing at the Sleeping Giant Inn in Riverwood, you may occasionally here a discussion between himself and Delphine (prior to the quest Alduin's Wall) that goes like the following:
Delphine: "Sven, we need to talk..."
Sven: "Here we go..."
Delphine: "It's about your singing. Well, not your singing, your songs. But you know what I'm talking about."
Sven: "You do realize these simpletons wouldn't know real music if it bit them in the britches, hmm? Very well. Your wish is my command."
Delphine: "Wonderful. And let's not have this chat again next week. Agreed?"
I emphasise the line from Delphine; But you know what I'm talking about. What IS she talking about?
Based on the response from Sven is implies she feels the locals dislike the song choice, however the songs he plays are no different to any other bard in Skyrim (Ragnar the Red, The Age of Aggression and Tale of the Tongues). Plus this discussion is had when Whiterun hold remains neutral in the war and "The Age of Aggression" is the political song of choice, so it can't be that his songs are anti-Imperial rule, not that she even cares that much about the war, though I don't imagine she would support the Stormcloaks.
I also don't think any of the songs are disruptive of her secret identity or overly difficult to understand for the local simpletons.
So, again; What is she talking about?