Here is a cassette tape demonstration of how speech can be enhanced using audio processing.
The original source was made in 1979 on a basic cassette recorder but suffered because of poor microphone placement.
Unfortunately many amateur recordings were spoiled because of this error.
With careful hiss and EQ adjustment a worthwhile result was achieved.
This process was all carried out with the addition of 'Cedar Audio' restoration and iZotope RX2 advanced software.