Hi, it happens with basically everything, games, music, youtube...every time some sort of sound comes out, it comes out fine, but after a second the bass disappear and the blurred sound mix with the rest of it. So if there's no bass, like in a whatsapp audio, the sound it's kinda fine, there is a little blurred noise but it's barely noticeable.
I also noticed that when any audio it's over, for like another second, the speaker makes this "bzzzzz" sound, i don't know if that's normal.
I really hope it's not the speaker itself, because the phone literally is a month old today.