[TylerE]

I seriously doubt this is true. Solving this problem is basically why BMI and ASCAP exist.

Where it gets tricky is that with a piece of music, there are at least 3 or 4 different elements that are separately copy written.

You have the basic tune, the specific arrangement, the physical layout and content of the sheet music, the basic copyright of a particular recording, and finally (and most relevant to your example), what are called the synch rights, which are the rights to use a song as a synchronized part of a larger work, like a movie or commercial.

[fencepost]

It's probably both - finding the holders for a lot of music is impossible, but by using BMI or ASCAP you may be able to shift the responsibility to do so to them in their role as PROs.

Not an area I keep up on, but this is interesting regarding how things worked in 2016 and possible recommended changes. https://www.copyright.gov/policy/pro-licensing.pdf Note that it's about Performing Rights Organizations not about Professionals. Of particular interest is the discussion of 100% licensing vs fractional licensing.

[TheHappyOddish]

These both appear to be American businesses serving American users. 95% of the worlds population don't live there. Both copyright laws and rights holders vary significantly around the world. This is far from a solved issue.