Sometimes the sound when a new email arrives works other times not.

Works sometimes on some mail account but not on others.

Are others having this same problem?

Tried rebooting the iPhone. Sometimes the sound starts working then stops again.

Strange, it's working for me every time...

Work OK for me all most of the time on one email account.
The others only sometimes.

Works fine for me too, even better now I've jailbroken and can actually change the damn thing to something worthwhile.

Seems I'm not alone looking at the apple forum's.

Various sound issues seem to be happening including mail notification.
The fact that lack of sound seems to move around the various mail accounts I have makes it appear software. Usually works on at least one account at any time but not all. I expect a software update will fix it.

Another bug appears to be the screen suddenly brightening up for no reason with auto brightness turned on. (using inside not change in lighting) Software I would think too.

It does appear that various problems such as these effect some but not al users.
As to why is a mystery.

If you have rules setup for your emails to skip Inbox and go to into other folders then you'll not hear email notification on your iPhone until you go to that folder and refresh to check for incoming emails. This is exactly how it works for me. I get instant notification for inbox emails but other emails that go to different folders don't notify me until I manually check them.

Not sure I understand all this. But. All the mail accounts were set up automatically from mail on the Mac. And that works ok. Set up is as imported to the iPhone with no changes made. Each account has it's inbox, sent folder, trash. No other folders created on either iPhone or Mac in mail.

Even if the set up was the cause the sound on receiving mail should stay in the one account, and not move randomly between accounts or stop altogether. That's why I thinking it must be software.

There is a bug were you sometimes don't hear notifications.. Eg SMS sounds.. Not sure about mail.. This problem pisses me off to the max.. I could be having a huge convo with someone via SMS and walk away from my phone for a second and not hear the SMS sound.. only the Vibrate sound.. I did alot of research and found heaps of people with the same problem.. I hope the next update fixes this problem

Same here. So much for push email.

The only workaround I know of for MobileMe / .Mac is to shut down Mail.app on your Mac.

That way the rules aren't applied (Rules don't seem to work at the MobileMe server level, or on the iPhone, despite selecting sync Mail Rules in MobileMe prefs on your Mac.

Thus all incoming emails will stay in the inbox, and will push to the iPhone.

Looks like you don't have any rules setup. So it might be a firmware issue..

2.1 did nothing for me regarding the mail notification sound missing most of the time.

Mail in general seems o,k but does have an issue going through the various SMTP servers, as per the instructions on the screen. Still a few bugs in mail it seems.