Fireblocks are always horizontal, and always 1 per stud bay. If there are more than one, they're probably not fireblocks, and put there for another reason. They're usually at chest height, because that's the easiest height to install them, for the framer.
How high off the baseboard are these 'fireblocks' located?
Flexbit - not what you pictured. Since you've since watched youtube vids, I should assume you now know what they are. But I won't:
Depending on the height of the blocks, you may be able to hit them from below with an 18" stiff drill bit - much easier to use than a Flexbit.
Cutting small holes behind the speakers will make your life easier, whether you come from below, or from the side (from the TV). The holes will be covered by the on-wall speakers. Make the hole wide enough so you can slip your phone camera in, to snap a pic of the 'fireblock'/obstruction.
You can probably hit the 'fireblocks' from the speaker holes, with a stiff bit, drilling down. Make the speaker drywall holes near the bottom of the speakers. Measure your speaker wire to determine the bit diameter.
It's much easier to drill down than up, with any bit, but especially Flexbits.
Your speaker wires will come out of the wall down below, behind your equipment. You could use single gang brushed/scoop/nose pass-throughs for those, at outlet height.