My latest test results:
Outgoing mail sound (whoosh) always sounds.
No sound for Get Mail if there is no mail.
If there is incoming mail, sometimes the incoming mail sound plays; other times it does not. Trying to determine why this is.
If there is mail but the incoming sound does NOT play, then deleting or junking that message produces the incoming mail sound. Replying to the message or Forwarding it does not produce the incoming mail sound.
I always check for mail manually.
Added note: if incoming mail sound does not play, it appears that deleting ANY message produces the incoming mail sound.