The active version could be something like this: "At midnight, the sleeping British were woken by the sound of German soldiers singing carols in their trenches." Note that both 'being' and 'singing' are present participles, rather than gerunds.

Being sung is a passive verb. The Germans were singing songs. Active The songs were being sung by the soldiers. Passive. A gerund is a verb that acts as a noun. This is not the case.