DIAGNOSIS This is usually a mismatch between your TV's audio output format and what the Beam is expecting, or Night Sound/Speech Enhancement being off.
FIX In the Sonos app, go to Settings > System > [Your Beam] > Audio Compression. Set it to 'Compressed' if it's on 'Uncompressed'. This boosts quieter sounds. Also, ensure 'Speech Enhancement' is turned ON. You can toggle this quickly from the 'Now Playing' tab by tapping the star icon.
IF THAT DIDN'T WORK Dig into your TV's audio output settings. If there's a setting like 'Digital Audio Out' or 'Audio Format', change it from 'PCM' or 'Auto' to 'Dolby Digital' or 'Bitstream'. The Beam is designed to decode compressed formats like Dolby Digital, and PCM can sometimes come through too quiet.
HEADS UP If you're using an HDMI switcher or an older AV receiver in the chain, it might be stripping out the center channel data where most dialogue lives. Try connecting the Beam directly to the TV's ARC port to rule that out.