This is probably because you have set the volume limit at a very low level. When a volume limit is set, volume could not be increased to a level higher than this limit.

To reset the volume limit:

1. From the main menu, select [Settings]->[Sound settings]->[Volume limit].
2. Press the buttons to adjust the volume to your desired level.
3. Press button to confirm.

Note: Long exposure to loud sounds can be harmful to your ears. It is recommended not to set the volume limit at a very high level.