This has been happening intermittently with my S26 Ultra 1TB, on One UI 9.0 Beta, report sent to Samsung, its random and infrequent but noticeable when it does occur. Tried the self-help techniques (checking for dirt/dust in bottom speaker grille) it happens when i record video. It stopped after I reset the audio confirmation 8n Dolby Atmos (turn off, reboot, test, reset, reboot).
Tried the speaker test function - Run Samsung Members Diagnostics
Hardware Test
Open the Samsung Members app, select Support > Phone diagnostics, and tap Speaker.
Cannot definitively state that its cured yet.