How to copy ringtones to Galaxy Ace 2?

I try to customize ringtone, so I copied files to "/system/media/audio/ringtones". Depending on the app, either the copy is forbidden or the copy is ignored.

It is because system partition is always mounted in read-only mode by Android. To edit anything in that partition you'd have to remount the partition in read-write mode. And in order to remount the system partition you would need root privileges. Once you've them, see the answers for Getting mount's usage suggestion when attempting /system remount or Can't remount /system in rw

I created directory "/mnt/sdcard/media/audio/ringtones" and copied the files there. This worked, however Android does not pick them up when selecting audio for a ringtone.

For some reason unbeknownst to me, if you move sound files into the Ringtones directory you would've to restart the Android in order to see those files as available ringtones through Settings app.

That however, is not the case if you use an app to set the ringtones.