This is getting out of hands. the normal speed audio clearly says " Sein " not " seinen " unlike the slow audio where " seinen " is clear. this is infuriating. I suggest that you hire someone to record those sentences/words instead of a computer. it's not hard to do and it's certainly not expensive to hire a native speaker to say simple sentences like these.
Because Bier is neuter, not masculine.
Only masculine things have separate forms in the accusative; feminine, neuter, and plural things are the same in nominative and accusative.
Thus, sein Saft in the nominative and seinen Saft in the accusative, but sein Bier could be either nominative or accusative, and similarly seine Cola.