Sorry for this noob question. I'm planning to play instrument and sync/edit it with the original song/record and upload it in youtube. Im just wondering if this is illegal or not. Thanks you in advance for those who will enlighten me.
If the original song are recording are copyrighted, then it is most likely illegal. And you'll have a hard time finding any recording that isn't under copyright.
More specifically, if the recording is was published in 1972 or later, or if it is unpublished, it is very likely protected by copyright under US law. In other countries, the date will be different. But in any country, any moderately recent recording is is almost certainly protected.
If the recording is protected, and you upload a copy without permission, with or without a synced performance of your own, you have committed copyright infringement This is illegal, but is not, in most countries, usually treated as a crime. Instead it is a tort, a civil wrong. The copyright holder can sue you and collect some measure of damages, even if you didn't make any money on the song.
In addition, YouTube (and other hosting platforms) have a system whereby the copyright holder can report the infringement, and get your upload taken down.
However, if you do not use the protected recording, but instead record your own version of the song (a so-called "cover version"), you can, under US law, obtain a "mechanical license" to use the copyrighted composition. This involves notifying the copyright holder, and paying a small fee. The fee is set by law, and the holder has no choice to refuse. This is all under US law, but I believe that other countries have a similar procedure available. Following this procedure would make your upload fully legal. Note, however, that a mechanical license is not available for any use of an existing recording, not for unpublished music.