Wolf,
I don’t see how the two cases are at all the same. By aveilus there would be an issur to proactively listen to music (For enjoyment). In a taxi or store you are not “listening ” to music although you may hear it.
If this is a case of KI ervah, the issur would not be to listen, it would be to hear.
To make it a little easier to understand, imagine if it were a naked individual, you would consider what you see ervah regardless if you went to look or it was there and you were seeing it, both would be ossur. (Depends on whether a recording of kol isha is ervah, but if it is, then there is a very clear difference from aveilus)