Intellectual Property (IP) in music, as in all creative work, exists at the point of creation. All music IP is the same. There is a common belief that a jingle or sting or a piece of sonic branding might be different due to its brevity but this is not so. Sound signatures or mnemonics, as they're known, are simply short pieces.
Rights in music are automatically assigned to a Performance Rights Society for safe-keeping as it were, in order that performances can be tracked properly and writers fairly compensated for their performances. Whether it's a 2-second sting or a 2-hour symphony, it's all the same (though naturally, the length determines the amount, as does the medium).
This system has been in operation for a hundred years (literally) for a good reason: royalties provide an essential income stream without which the music industry would struggle to function. It is not a free pot of money, up for grabs by whoever cares to claim it (as some would lead us to believe), but a legitimate payment for writers and publishers providing a quantifiable and bonafide service.
There is a widespread misconception about how royalty payments are made and who makes them. Some clients (and even some ad agencies) believe they are responsible for them but they are wrong. These payments are the sole responsibility of broadcasters and/or content providers who pay a levy (a 'blanket licence') to A Performance Rights Society based on the consumption of music on a channel/medium over a given period. It never involves the clients, agencies or film production houses. Ever!
Despite this, some clients still want to obtain IP. They might want to avoid having to deal with moral rights, for example, which restrict how a piece of music can be modified without the writer's consent. They might want to have the freedom to use the work for a lifetime without further licensing payments.
This is all fine from a Marketing Director' or lawyer's perspective but obviously not so form a writer's standpoint. To assign rights to someone else, a writer would have to pretend that either it wasn't theirs in the first place or it simply didn't exist; and why would they do that?
The other solution might be for a client to make a writer an offer which is at the very least equivalent to, or hopefully in excess of, the royalties the writer would have lost in giving up their IP for up to 70 years after their death. But this would have to be a very large sum of money indeed; and why would a client want to pay that?