Just off the top of my head, here are some variables you can't account for in a recording, and these items would effect just the frequency response, little lone not being able to tell anything I stated above about a system from a recording:

- Accuracy of recording mic
- placement of mic (RTA your Car and then move the mic 3 inches, the Frequency responce will be different)
- quality of source music
- amount of file compression in said video
- any EQ Implemented (if trying to evaluate speakers alone)
- enclosure size and effect of said size (if trying to evaluate speakers alone)
- "The Room" and it's effects on said speakers (if trying to evaluate speakers alone)
- Playback Speakers/amp/headphones/eq/etc. used to listen to recording (a person could use the same for everything they listen too, but the Frequency Response would have to be perfectly flat

And I'm sure I'm forgetting other variables.