Right-click on the account name, choose Properties and go to Sound. Turn 'Play sound when new mail arrives' on. When you have done that, you can also specify the desired sound file, as well as the times during which it may be played.
I have an update, thanks to the other thread about filters. Instead of using the POP sound feature, just set a filter to play a sound. With 7.4.8 this is an easy solution for playing sounds on incoming mail for IMAP.
What kind of sound do you hear? A system sound or a custom sound? If it is a system sound that resembles error sound, look at the account log (you can open it by pressing Ctrl+Shift+A in the main window) - are there errors in the log?
If there are no errors in the log, then maybe you have the sound to play configured in the filters. As you have shown in the screenshot, IMAP doesn't have sound options in the Account Properties, and the way to play sound in IMAP is via the filters. If it is the case, then you can remove the sound palying action from the filter.