I used to have the notification set to both popup window and sound, but have been forced to turn the sound off. This is because whenever I wake my machine from sleep or hibernate, eMClient keeps making the sound for each new email that it fetches from the server – so I often end up listening to 10-20 notification sounds in a row.
I think it would be great to have the option to make the notification only sound on the first of several incoming messages – perhaps implemented by not making the sound if the previous notification was (for example) less than two seconds ago.
This way, I could still have the helpful sound turned on but not be bombarded with them when the machine wakes up.