I assume you are flipping between OpenATV and ViX?
The Multiboot plugin just loads an image from external storage and doesn't change anything other than the multiboot hooks.
As has been reported in several threads there was an issue with the SF4008 drivers ... hence the audio issue.
OpenATV build every night and so their image has the fixed drivers, as long as you use a recent image. OpenViX will have the fixed drivers when the next image is available.