Marco Arment has released a version of his Overcast podcast player with “Voice Boost 2,” if you are interested in the nerdy audio side of this, he’s written up a blog post explaining it:
This is about to get nerdier, but bear with me. (Yes, nerdier than ITU broadcast-loudness standards.)
Given a loudness measurement for the incoming audio, quieter podcasts need to be amplified to reach the target. But perceived loudness isn’t the peak of the incoming audio stream — it’s more of an average. Quiet-sounding audio can still have brief moments of loud peaks.
When increasing the volume of digital audio, the biggest challenge is not “clipping” during the peaks — not having any part of the signal pass above the volume ceiling of 0 dB. (It’s a negative scale. This is also why the LUFS value above, which is closely related to the decibel scale used here, is negative.)