I resolved it by going into mail preferences, general tab & changing the sounds to "none" and unchecking "play sounds for other mail actions".
Then I quit mail from the top menu bar. I restarted mail, went back into preferences did the opposite: changed sounds to "new messages sound" and checked "play sounds for other mail actions", I then quit mail again from the top menu and then reopened it. Magically, my whoosh was back.
Sometimes it's easier than you think
I have the same problem with my brand new Mac Book Pro. I have spent hours and hours with Apple Care and they have tried everything under the sun and it hasn't worked so far.
Restarting the computer or connecting to Airplay/Apple TV seems to help, but my mail sounds sometimes work and sometimes not. I am so frustrated!
Still no fix.
Being able to get sound notifications is important for me, because when I'm away from the computer I need to hear when an email message comes in. Some of these messages are very important and require a quick reply.
I have sent feedback to Apple using the "provide mail feedback" option in the Mail menu.