Hi saeed.ayman,
Thank you for using the Apple Support Communities! We understand you're noticing a change in volume on your Mac. We'd like to help.
-Is this happening only when using a specific application? Or, are you noticing this in any and all apps?
-Are you listening through your Mac built-in speakers? Or, are you using an external speaker or a set of headphones?
-Does the same issue occur if you connect a set of headphones or other external speaker?
If this is happening when using the Music app on your Mac, check under Music > Preferences > Playback, if you have Sound Check enabled.
Keep us posted.
Regards.