There is a built-in setting that covers part of this. Go to Settings > Sounds and vibration > Volume, tap the three dots at the top right and choose Media volume limit. Turn it on, set the highest level you want to allow, and use "Set volume limit PIN" so the limit cannot be changed without the PIN. Keep the PIN somewhere safe: if it is forgotten, the only way to clear it is Reset all settings.
Two limits worth knowing. It applies when headphones or a Bluetooth speaker are connected, not to the phone's own speaker. And it caps how loud media can go rather than freezing the volume at one exact level. If you need a true lock, for example for a child's device, it would help to say which case you have in mind, so Samsung can see what the request is for.
