What I expected to happen was when going through a portal to the Nether, after you have left the Overworld, on the download terrain screen, travel music is supposed to play. (travel.ogg)

What actually happened is no music played.

To reproduce
1. Enter a Nether Portal
2. Wait about 5 seconds
3. When it says downloading terrain, travel.ogg should play.

This bug seems to have been around for a long time.