Having only one speaker is bothersome in that if the speaker is blown, unless it rattles, it may be difficult to diagnose.
Does this device have an Adaptive Sound setting, or any other sound profile options? Changes to the sound profile can have unexpected and undesirable affects. Things from go from punchy and rich to distant and soft, or even tinny. You can search for the term, sound, in your settings search bar, and look for anything that sounds like it would change your sound profile. It might even be called Sound Profile, or Equalizer. Maybe an update changed a setting or something.
Barring this being a setting thing, or there being debris in behind the speaker grille, which is unlikely on most devices since many speaker grilles are a fine mesh to prevent things like this from happening, it sounding more and more like a blown speaker. Does the sound you do hear sound distorted, especially if you max the volume out? Or is it clear, just at a much reduced level?