Thanks for your suggestions so far!
Meant to add that I've tried (unsuccessfully):
1. Variety of image resizes (small to original) and variety of different images
2. Connectivity on fast wifi and slow 3G
I noticed that on uprgading to IOS6 that the Whoosh happened immediately after hitting send, where as prior to IOS 6 the whoosh would only sound after the email had sucessfully been received by the mail server.
Any other thoughts?
Have you tried restarting or resetting your iPhone?
Restart: Press On/Off button until the Slide to Power Off slider appears, select Slide to Power Off and, after the iPhone shuts down, then press the On/Off button until the Apple logo appears.
Reset: Press the Home and On/Off buttons at the same time and hold them until the Apple logo appears (about 10 seconds). Ignore the "Slide to power off"