I don't think I have ever seen a specific sub cable for in wall use and no you can't joint hem unless in an accessible junction box. I think you could just use RG - 6 coax cable for your sub cable and then termminate it at the box with an F type to RCA adapter in a wall plate.
Plates to come after drywall finished and painted.
The cable that I linked to originally is a 50 footer and is CL2 rated for in wall use.
Regarding the j-boxes. Make sure you get low voltage boxes, with open backs, they are much easier to work with (trust me, I know).
Edit: Zarack, you can get next day air from monoprice.com for $35...even at that price, the 50 footer is worth it. I would suggest getting a 6 footer of the same cable for when you get a wall plate up...you should look around the site and see if oyu need any other types of A/V cables and bundle it all into the same order.
Monoprice also has this statement on their main page "California Overnight Sunrise, delivered on the next business day before 2:00 PM Pacific Time for Only $7.50 Flat upto 10LBs, is available for CA, and Reno, Las Vegas, Pheonix, and Tucson in NV and AZ. Please consult the Service Area and ZIP Code Guide for additional information."
