The game mode is not specific to the bug.
How to reproduce.
1. Go in 1.5
2. Listen and wait for calm4
3. Realize there is no calm4 playing or will ever play
Comments 5
If there was a calm4.ogg at all, it hasn't been around for a while. I remember because I went in during beta, I think around beta 1.7, and copied all the existing ambiance and converted them to .wav to listen to in another game. Looking at the sound files now, there was no calm4.ogg
There was a hal4.ogg, however, that sounds similar. You may be confusing the two.
There is a list of all music in the game here:
http://www.minecraftwiki.net/wiki/C418#Music_included_in_Minecraft
That file was removed: http://www.minecraftwiki.net/index.php?title=Calm4.ogg
Ah, yes, I remember this. In the old assets system (under the old launcher I believe) it would randomly select music from the folder, and if you had run the game when Calm4 was a thing (or, manually downloaded it), it would play. This was a thing in the far past, but broke with the new launcher.
You could re-add it with a resource pack, though... hm...
Please do not mark unreleased versions as affected. You do not have access to this yet.