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A few things:
Depending on how high the speakers are being mounted, it might be easier to use a 5 or 6 foot long 3/8" drill bit. The flexible shaft allows you to cut a box opening in between studs, insert the bit in the hole, and drill through the bottom plate and subfloor. Once the bit is through, there is a hole in the tip running through the drill flutes to which you attach the wire. Remove the drill bit, and the wire is now pulled. Use caution! The bit is no smarter than the operator, and you can eff up plumbing, electrical, and other utilities in the wall if you're not careful.
Speaker wiring is considered low voltage, so it doesn't need a full device box to mount the speaker plate. Carlon makes low voltage 'rings' for old work in one and two gang sizes. Complete your drywall cutout, slip the ring into place and tighten with a philips screwdriver. Then you can attach a device plate with either a hole, or if you'd like a more professional look, get plates with red and black spring terminals on the face. Terminate the wires on the back side, and then you can cut just what you need to go from the wall plate to the speaker.
Locating the bottom of the wall from beneath is likely to be a bear. Most framing sits atop the deck sheathing, so your only cues are going to be other utility penetrations. 3 1/2" doesn't leave much margin for error.
Good luck
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There ain't nothin' more powerful than the odor of mendacity -Big Daddy
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