Someone in the room suggests a song everybody knows. It fits the cut, the client likes it, and the question becomes how to clear it.
The short answer is that you need two permissions from two different people, and either of them can say no.
Permission one: the musical work
The composition and the lyrics are one copyright, owned by the writers or whoever they assigned it to, usually a publisher.
Putting music to picture is a synchronisation use, and it needs a sync licence from that owner. This is the permission people forget exists, because nothing about playing a song in a room suggests that syncing it to a film is a separate act.
Worth being clear: this permission is needed even if you re-record the song yourself. A cover version replaces the recording, not the composition.
Permission two: the recording
The specific recording everybody is picturing is a different copyright again, owned by whoever produced it, usually a label.
That needs a master licence. If you only have the sync licence, you may re-record the song but you may not use the famous version. If you only have the master licence, you have a recording you are not allowed to synchronise.
You need both, from both, and they are rarely the same party.
Where IPRS fits, and where it does not
IPRS licenses the public performing and mechanical rights in literary and musical works. Synchronisation is a different permission again, and a performing-rights licence does not stand in for it.
What it does not tell you is who grants the sync permission. That depends on what the rights holder assigned to the society, and the membership deed takes performing and mechanical rights broadly. So confirm it with IPRS and with the owner rather than assuming it sits in either place.
Assuming is the expensive part, because the assumption surfaces after the film is cut.
What this costs you that is not money
Time. Two parties, each with their own process, neither of them working to your edit schedule.
Certainty. Either can decline, for any reason or none, after you have cut the film around the track. There is no appeal.
Scope. A licence is for a stated term, a stated territory and stated media. When the term ends, the campaign stops running. If it was cleared for digital and the client wants television, that is a new negotiation from a position of no leverage.
A ceiling on reuse. The track you licensed for this film is not available to you for the next one at the same price, if at all.
When commissioning original is the shorter route
Not always, and anyone who says always is selling something. A song the audience already has feelings about is doing work that original music has to earn.
But the comparison is rarely like for like. A commission has a known cost, a known delivery date, and nobody outside the room who can refuse. It can be written so the advertiser owns the music rather than renting it, which means no expiry, no territory limit, and no second negotiation when the campaign extends.
The paperwork for that is one document rather than two, and it is worth getting right: paying for music is not the same as owning it, and the clauses that decide which you got are the ones most often left out.