Having trouble with sound effect in Mail App

I have an iMac Retina 5K 27" produced in 2017. It is currently running MacOS 10.14.6

I have this problem with the mail program. The sound effects, mostly for when an email is sent, don't work. More correctly, they work, then for no reason I can fathom, they stop working. A complete shutdown and restart will get them to work again briefly.....but then they stop working.

Minor problem, to be sure. Especially since sound is fine when watching videos on YouTube, for example. But it is a bit maddening....and was wondering if the community at large has any ideas?

Thank you.