Imac makes this bubbling sound to alert me on emails, pushed notifications, etc. How do I make this stop?

Go to System Preferences>Sound>Sound Effects and choose a sound that you like.

No, the bubbling sound which I hear on occasion on my iMac does not correspond to any of the sounds in System Preferences. So I don't think that "changing" the sound in System Preferences will have any effect.

I forgot to say that the source of the bubbling sound (for me on my iMac) is Skype. The sound signifies "Contact becomes Available". You can either turn off or change the sound in Skype Preferences.