Money for old tracks: a reader asks

One of our readers has written to ask the 1709 Bloggies if they have any advice or experience that they can bring to bear on this little problem.

A British-based advertising agency has been approached by the American Federation of Musicians (AFM) in relation to its use of US music tracks. It seems that the AFM is asking for backdated payments on behalf of background artists who contributed to those music tracks, for example as backing singers ot musicians.

This agency has assumed that the normal procedure adopted by agencies in the UK when they want to use music tracks is (1) contact the music publisher and (2) contact the recording company to seek permission to use the sound recording. When it uses British music, it generally contacts the Musicians Union, which confirms how much to pay with regard to background artists. However, where US music is concerned, this agency has not been contacting the AFM, because it didn't know it had to -- a belief that was reinforced by assurances from some of the recording companies. The UK agency has also heard that the reason why advertising agencies in the US contact the AFM to arrange for payment in relation to use of background artists is because they have already made an agreement with the AFM to do so.

The UK agency therefore asks as follows:
(1) if it isn't a signatory to the agreement that was signed between the AFM and US agenices, does it still need to pay?
(2) does the answer to (1) change if the practice for some time has been that it never had to pay? and
(3) if it does have to pay, will it have to pay the backdated sums? and
(4) likewise, if it has to pay, what proof if any does the AFM need to bring in order to establish how much it should pay (right now, it seems, the AFM is asking for what appears to be an arbitrary sum, based on little proof of how many background artists were actually involved in a track).

The UK agency adds that it has heard that advertising agencies in Canada are facing the same payment requests from the AFM.

Can any readers offer any practical or legal guidance here?