Good question. Have you asked Westbahn?

Westbahn is not on the list of included railcompanies on the German Rail Pass website, but the best is to ask them.

https://www.bahn.com/en/offers/passes/german-rail-pass

On their site: Interrail is valid if it is valid in Austria (afaik Global pass or Austrian Onecountrypass)

no mention of Munich-Salzburg with German Onecountrypass. 

This is just a sort of advanced speculation:

your pass is in DE valid on about all trains, not just DB but also the dozens+more of other companies running local trains in specific areas. There is a general all_DE tariff for that. But most passusers will only use the IC/ICE trains from DB and likely, as its so hip, the NJ overnight trains-all go to Austria. Note that DB also has quite a lot of overnight IC/ICE trains, these offer seats only and make multiple stops in the night-not that safe or comfy, but doable.

SOME trains are out of this system, that is mostly the green FLIXtrains-linked to yes indeed, the green FLIX buses, and a very few local trains that you will be hard to find, but there are always determined railfans.

THis brand new WB extension to M does NOT accept the general DE-tariff tickets on this sector, quite different from the original sector in AT. SO I conclude that a pass will also not be taken on it. But as above-its evident that the comp itself, and they are customer friendly, will give best answer. And what is now can also change later.

WB also plans to extend other trains now ending in Sbg to Tirol/Innsbruck, via the short-cut-off route also via DE, as to how/when/what passes will be valid there is also by now a mystery-if someone takes the trouble to ask them, ask this right away!

I got an answer on this specific question from Customer Care Center of Westbahn.at (Mr.Böhm) : interrail is valid!

Indeed, and that's why I refrain from speculation.

rvdborgt.  you gave the correct and concise advice! I did it with succes! 

@Eurofima did you mention that you have an Interrail German Rail Pass? 

I don't have a Germanrailpass myself, I asked for someone I know.

My question was: 

Geehrte Leute von Westbahn

Ist ein one-country Germanpass interrail gültig in Westbahnzüge zwischen München und Salzburg? Normalerweise sind alle andere Züge benutzbar.

The answer was: 

Geschätzter Kunde,

 Grundsätzlich gelten Interrail Ticket bei uns. Unter den Richtlinien von Interrail.

Mit besten Grüßen