Dealing with muffled audio remains a common problem for sound engineers and home users alike. Muffling results from the lack of high frequencies in a sound wave. The same phenomena occurs when you hear someone talking through a wall or door. Equalization is the solution. This process allows you to boost certain frequencies while decreasing overbearing frequencies. Audio recording programs come equipped with equalizers.
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.
An equalizer may simply be referred to as an "EQ" in some audio programs.
Restoring audio is a refined skill and takes great care and experience to achieve professional results. Each piece of audio is unique and requires adjustments specific to the amount of muffling in the audio itself.