Yes, it's normal for the Samsung Galaxy Z Flip 5 to vibrate slightly, especially in the top half where the speakers are located. This vibration is a result of sound waves produced by the speakers, which can cause the phone's thin and compact structure to resonate. Since you've already tried adjusting playback settings and turning off Dolby surround without a change, it indicates that the vibration is inherent to the phone's design. As long as the vibration is not excessive or accompanied by rattling, it should not be a cause for concern. Do speak with Samsung directly as this was something I had to look up online about.