Open the muffled audio in your recording program. Select an open effects insert. A drop-down menu will appear on the track. Select "Equalizer" from the list.
Adjust the knobs on the right side of the Equalizer so that they read "+3db". This increases the high frequencies by three decibels. This amount can be adjusted as needed. High frequency controls are located on the right side of the equalizer and descend to the lower frequencies on the left.
Listen through your audio track. If muffling persists, turn down the equalizer knobs on the left side of the equalizer to "-3db". This reduces the bass frequencies of the audio.