The OP can correct me if I am wrong, but appears they are talking about this weekend. There is track work indicated on the Munich-Salzburg rail line between Friday June 7 and Thursday June 12, between Teisendorf and Freilassing.
They are also showing alternative service using regional train to Muehldorf and a connecting train to Salzburg, taking 2 hours and 47 minutes. This route is marked with the red triangle, and noted at the bottom of the page:
Mühldorf(Oberbay)->Salzburg Hbf: Information. Exceptionally high number of passengers! Travellers who have not yet purchased a ticket, please choose another connection. Hindrance probably lasts until 12.06.2019.
The regular Meridian service shows a replacement bus between Teisendorf and Freilassing. There is no such note on these trains, but it takes longer to get to Salzburg because of this, 2 hours and 21 to 27 minutes instead of the normal 1 hour and 46 minutes.
My suggestion, just take the regular Meridian service, as there is no restriction noted on these trains, it will just take a little longer to get to Salzburg. So if you take the 7:53 train at Munich Hbf, you get to Salzburg at 10:19. At Teisendorf, everybody will get off the train and board waiting buses and then be transported to Freilassing where they will board OEBB trains continuing on to Salzburg.
Return will be the similar process. Trains at 17:42, 18:46, 19:42 (with extra connections), 20:44 (with extra connections) etc.
Net result, just go and do it. 1st class is unlikely to do you any good.