Thanks to everyone for their responses. I'm yet to read anything from a PMDG user and Soundstream user which confirms the question one way or the other though! Yes, I could buy Soundstream and try it but there seems to be so much doubt as to whether or not it'll work for me sufficiently well. It seems silly to buy it knowing that it may not work. I don't want to rely on the no quibble money back guarantee.

Well we can't help you anymore, as I said already some sounds will still mute when using the NGX or 777, but without you trying it for yourself and listening to the result you cannot possibly come to a conclusion whether its suitable for YOU or not. If PMDG implement an option to not mute their extra sounds then if will not sound any different regardless of whether FS has focus or not, IF you have Soundstream of course.