Different programmes and sources can have different mixes and loudness; a channel change is not a controlled test of one fixed audio signal.
Why this happens
A quiet drama, a studio interview and a sports broadcast do not necessarily arrive with the same perceived loudness. Their mixes can differ even when the television’s volume number stays unchanged. The player may also select a different audio format or soundtrack after a channel change. That is why a sudden change in volume should be described with the two sources involved, rather than treated automatically as a remote-control or account problem.
Begin at a comfortable volume and compare the same passage when testing a setting. If the device offers a documented loud-sound reduction or dynamic-range feature, evaluate it on material you know rather than assuming it normalizes every channel. Such a control can change the balance between quiet and loud moments, which is a different task from selecting the correct language or repairing silence. Keep the source, chosen audio track and soundbar mode with your comparison. Avoid turning the volume far up on a silent source, because sound may return at that higher setting.
A useful way to work through it
Compare two named sources at the same normal device volume.
Note any audio-track or output-format change when switching.
Try only a documented loudness/dynamic-range feature and judge its effect on a familiar passage.