Posting here as can't se a Sound Forge speciofc forum.
I have SF studio 10 (full version)
I have many video projcets in MSP with 5.1 audio tracks, and thought the following would be correct approach ... in MSP 'render as' Dolby AC3 5.1 to get an audio only file,
Import that into SF and then do any editing I want and then save as AC3 5.1
However I do not see any option to save as AC3
Do I have to 'Save as' some other format supporting 5.1 and import that back into MSP to rerender as AC3
The Studio version of Sound Forge probably doesn't come with the Dolby Digital AC3 encoder. Besides, you do not want to render to that and then edit it. Dolby Digital AC3 is a delivery format not an editing format. I believe you need to purchase DVD Architect Studio or DVD Architect Pro to get the Dolby Digital AC3 encoder as it is only used as a DVD delivery format for audio.
You want to do all of your editing on uncompressed PCM wav files.