The 30 seconds isn't a standard, it's just the length of time the copyright holder feel is acceptable for a sample listen – and it's just for a listen, to evaluate it's suitabilty. There is no attached right to use it in any way at all. If you wish to use ten seconds worth of a copyright work, you need to get it cleared. The famous example is Fleetwood Mac's intro to the Chain – the motor racing theme. Nobody ever plays the rest of the song, but you cannot use that bass guitar riff without paying.
If you are making a documentary on somebody, it seems risky to pinch material from the subject!
If you google PRS and PPL, these will be able to clear whatever clips you select – cost depends on distribution and quantity.
