If you are in the UK or the US, the answer is that the recordings are totally the property of the client.
If you have WRITTEN permission to use the song or other material, then do so. The two issues are totally separate.
You may (under English and Scottish law) give reasonable notice that the recordings will be destroyed, unless the client comes to pick them up. You have to give the client reasonable notice. Under Scottish law (and I believe under English and US law, but you will have to check this!!!) you may put a lien on the recordings, until such time as the debt is cleared.
Lien - Wikipedia, the free encyclopedia
When you give notice of the lien, you can also give notice, that, unless the debt is cleared, you will seek to use or exploit the material to recover the debt.
In the case of a car repair, a workshop gives notice that unless the debt is cleared within a set period, the car will be sold to recover the debt.
You will have to however, research the law on this issue and the effects of copyright issues of exploitation of the material in your jurisdiction.