Aside from blocking visual cues (lipreading etc), masks also reduce high pitch sounds essential to conveying speech information.
The data show that each mask essentially serves as a low-pass filter, attenuating the high frequencies (2000-7000 Hz) spoken by the wearer, with the decibel (dB) level of attenuation ranging from 3 to 4 dB for a simple medical mask and close to 12 dB for the N95 masks.
Source: How Do Medical Masks Degrade Speech Reception? - Hearing Review
For those who are in communication settings where face masks are being worn regularly, consider asking your audiologist to create a face mask program on your hearing aids… the setting should try to accommodate for the frequencies that are filtered by the mask by adding extra amplification to those pitches.