DIAGNOSIS This is a dynamic range compression issue. The soundbar is receiving a high dynamic range signal (like Dolby Digital) meant for a theater, not your living room.
FIX Press the 'Sound Mode' button on the remote repeatedly until 'SMART' is no longer displayed. Use the 'NIGHT' mode instead. Night mode applies heavy compression to balance loud and quiet sounds. Also, manually increase the Center channel level in the JBL One app under 'Advanced Audio Settings'.
IF THAT DIDN'T WORK Force your TV or streaming device to output a compressed audio format. In your device's audio settings, select 'Dolby Digital' output (not Digital Plus or Atmos) and enable 'Dialog Enhancement' or 'Night Mode' if available.
HEADS UP If dialogue is muffled even at high volume, the center speaker driver may be damaged. This requires disassembly to confirm.