The mixes are fixed video streams with an interactive overlay that swiches between the audio tracks, so a separate one has to be created for each market. So far they've only created local (ABC/CBS/FOX/NBC), weekend sports (CBS/FOX/ESPN/FS1) and Olympic mixes (NBC/USA/CNBC) for the 29 markets listed earlier in the thread. That 30th one with the national NBC stream ended up being just for Tampa, Austin TX and Columbus OH where MySports customers get the national NBC stream instead of their Nexstar owned affiliate.
There's no obvious reason why they started with those 29 markets and skipped some larger markets. i.e. Philly doesn't have one even though their Big 4 are all O&Os like NYC, LA, Chicago and San Francisco. Boston (Hearst ABC, CBS O&O, Cox FOX, NBC O&O) was also skipped, even though other Hearst and Cox markets have mixes.