Most music used for these types of videos is either a public domain work, which could include expired copyrights, or music that has been categorized as royalty free and is free to use for all publishing or filming purposes once the membership cost is paid.
This can actually save you a lot of time and trouble, since most musical ditties and samples are owned by their creators and they will usually want royalty payments and perhaps approval of whatever your project may be. It’s understandable why directors will save themselves the trouble and choose background music that is ready to go, without any special conditions.
Music for an introduction cannot be distracting from the subject matter or inappropriate for the message you want to deliver. For serious subject matter, more emotive and mood shifting music is used. For laughs or for a light commercial type of video, quirky and faster notes work better.
Back in the early days of Hollywood light musical scores were integral to the production since there was no talking yet. These musical pieces were carefully selected to reflect the mood of the piece, though some people preferred the aesthetic delight of a truly silent film with only images, and no sound taking away from the mood.
Usually when original music is scored for use in a film, the composer sees the finished product and then writes the composition to match the story length. If you are using intro music for YouTube videos and buying them royalty free, you can’t afford to do that; however, you can still edit and choreograph the story to match the visuals to the length of your selected score. You can also use audio repetition or “rests” to fill out the time.
Other tools used in a music production intro might include some non musical but still aesthetic sounds such as birds chirping in nature, ticking clocks, bells, a person’s heartbeat and even heavy breathing. These sounds perform the same function as a piece of music: to put the viewer in a frame of mind, calm them or call their attention. An mp3 / wav may be just what you need to give your videos the energetic start they need.