I haven't a clue how ま with a dakuten is supposed to be pronounced, nor what it means.

Arcana55 said:

I haven't a clue how ま with a dakuten is supposed to be pronounced, nor what it means.

Extra dakuten just indicate that something is said particular loudly, or sometimes nasally. Generally just to indicate shouting. For example, 「あ゛あ゛！」isn't too uncommon.

You see this in gitaigo or onomatopoeia pairs too, with the version with dakuten being larger, greater magnitude, or louder: 「ゴロゴロ」 is something larger tumbling (loudly), 「コロコロ」 is something smaller rolling softly.

Arcana55 said:

nor what it means.

It is Giant Robo (Tokusatsu)'s signature growl.