Double checking my math here.

5 Mbps / 8 = 0.625 MB per second
0.625 MB per second x 60 seconds = 37.5 MB per minute
37.5 MB per minute x 120 minutes (or 2 hours) = 4,500 MB or 4.5 GB

Audio should only add a miniscule amount of data per 2 hour movie since 1 hour of 128 kbps Opus = ~60 MB