When working with frozen audio tracks, there are some important things you can’t do, such as messing with Warp markers and adding new effects. Typically, the Flatten command is used to turn a frozen track into a standard audio track.
When a track is flattened, the original track is thrown away and replaced with a new Audio track containing the (formerly) temporary audio file created when it was frozen. So, while you get the ability to freely edit and add effects, you lose the ability to go back to the original track (there’s no “Unflatten” command!).
Depending on what I’m doing with a track, I’m a fan of creating a flattened copy of the track, rather than the original. This can be done in a couple of ways. Just like MIDI, you can copy a flattened audio or MIDI clip into an empty Audio track.
Alternately, you can use the Duplicate command on the frozen track, then flatten it.