Audio Boost allow to boost intelligibility of speakers who are very distant from the microphone or speaking very softly. Also reduce noise. It is a paid feature.

Step 1: First confirm you have purchased it from Settings Screen.

Step 2: Open the Voice Memo list and tap on detail button of selected voice memo.

Step 3: Scroll down to select audio boost mode.
High Fidelity: Ideal for recording music and speeches that require a minimum boost in volume
Standard: Ideal for most voice recordings where the main speaker is distant from the microphone.  Noise Reduction is turned on by default.  Turn off Noise Reduction for increased intelligibility.
Whisper:  Ideal for boosting intelligibility of speakers who are very distant from the microphone or speaking very softly.  Noise Reduction is turned on by default.

Now tap on Audio Boost button. A new file will create and save top of the list.

Video Demo: