This is actually the easiest way to get music to your One X/X+/One/One Mini/One Max, since they all uses MTP which is a part of the media player framework.

This works on ALL Android devices that uses MTP for usb connection.

EDIT:  this of course works on the new One M8 as well.

So, how do you do it?

1. Start Media Player, make sure you have the music you want to sync in there. Will not cover how you do that, as it’s rather simple.
2. Connect your phone, it will after a while show up to the right.
3. Simply drag the music to the indicated place, it will show you a list of what will be synced to your phone.
4. Click on Sync now, it you will show you progress of it.
5. And you’re done!