Sound and video when listening on my Melomania 1 are out of sync

Most users do not experience problems with latency, however if you do it can be down to a number of factors

Latency is dependent on audio Codec data transmission.
The wireless audio connection creates a slight delay from when audio is played on the device until the audio reaches your speaker. This delay becomes noticeable when playing audio that is part of a video. This is similar to many Bluetooth® speakers and headsets. Which means that you may sometimes experience small latency between the video and sound.

Reset your product.
Much like rebooting a smartphone, your Melomania 1s might need to be reset on occasion to correct minor issues.
1. Place both headphones in the case.
2. Press and hold both buttons until the earphone status lights briefly turn blue.
3. At this point, you can release the buttons as the reset procedure is complete.
4. The earphones will no longer remember any of the devices that have previously paired with them.
5. You will then need to follow the initial setup procedure from scratch including the manual Bluetooth pairing process, achieved by holding down the earphone buttons for 3 secs to enter pairing mode.

Try a different app.
If the issue only occurs with a particular app, it is likely related to the app or app settings.