Audio is definitely a must have when on a call so I feel your pain! But don't worry because I'm here to help.
Are you having these audio issues with every call you make or receive? How long have you had this issue?
What settings are you referring to? Are you accidentally hinting the mute button when on a call?
Try rebooting the phone by removing the battery for a minute or two. Re-insert the battery and retest.
Keep me posted if you need further assistance.
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Just a thought did you recently drop your phone etc? My wife did this with her fascinate and now has the same issue and reboots etc are not help. Pretty much realized the drop affected the microphone somehow and once we get the IPHONE we will send in our warranty since it was covered with accidental stuff.