Currently as mobile support advised (as designed), the mobile droid app will only play the sending mail sound you selected "when the app is running in the background.

Now there doesn't seem much point enabling a sound notification for sending mail "when you are using the mail program in the foreground". So you currently have to press send and then go back to your Droid home screen to then get the sending sound notification to work. Then reopen the app to continue on with thr rest of your emails. Seems a crazy way to get a sending sound in a mobile mail app.

You expect the selected sent sound notification to play when the app is open "while you are still working in the app" in the foreground.

Yes it's good to have a banner notification that the message will be sent shortly, but if you enable to also have a sound for sent messages, you expect that you will also hear the sound after sending while open.