Is תְּשׁ (a ת with shva followed by a שׁ) a digraph for the "tch" sound, as in swiTCH?
It's not a digraph, as the correct pronunciation is "Tishuva" (a very short i after the T).
The audio is how people usually say it.
Could you refer to a source about this correct pronunciation? Seems a little weird
The ת gets a דגש קל for being at the beginning of the word.
Therefore the שווא under it is נע.