Will power cable work well as speaker cable?

You could, gasp, dissect the cable and get rid of the shield. Throw some tech flex over it and call it a day. If you don't like the shielded sound, that is.

Fancy seeing you here, Steve!

Steve (Willie) is right though, the helix cables sound amazing. Maybe not the best first diy cable project, but well worth the time and effort..

keep us posted on your power/speaker cable journey.

I've not tried it, but I've read many anecdotes that proclaim Romex to be an amazing speaker cable. But a bear to get into place. If you do use your 10/5 Romex, (you sure about that? 5 conductors plus a ground? I've never heard of that..) don't use the whole thing, just strip the conductors out that you need. 2 for regular cables, 4 for biwire. You might even want to try a helix configuration, based on williewonka's post above..

Hahaha! I had to read that last part three times..

Man, 2 x 10awg per polarity is a bit crazy. I mean, unless you’re pushing 2000 watts..

I’ve been making lots of cables and was loving some braided cat 5 cables. However, I recently made a pair of cables like williewonka’s link above, the helix. They sound great. They key parts of the great sound are the coil architecture and the double neutral. Check it out. It could sound amazing with Romex, but your hands will be sore, making the coils!
Whatever you do, those are going to be some stiff cables!
How the crunk are you going to fit all that wire into a binding post?!