Adjusting Volume on Extensions

Is it possible somewhere in Asterisk to increase or amplify the volume on a specific extension? We have an employee that has a hard time hearing his handset and his speaker phone even though they are turned up all the way on the phone. Thanks

As I know, You can change volume only at end points or where are analog to digital or digital to analog codification.
Change TX/RX level in setting of your ATA or HardPhone , or you can adjust volume on you mic in your soft-phone settings. Also you could configure tx/rx gain of your PRI or other FXS/FXO cards.

If you try and increase the volume on a VoIP phone, it will just clip and distort the audio.

You need to look into induction loop, Direct Audio Input, or Bluetooth coupling into their hearing aids.

Generally people who need hearing aids need different frequencies amplifying differently and the dynamic range reducing, so just turning up the volume, even on the handset, is not an ideal solution.