I wonder if that 'folk voice' was partly a product of having to sing without amplification? Not everyone has a powerful singing voice with a good range and the average folkie was probably not trained in voice projection, so making ones self heard in a large room full of people who maybe were not too quiet could be a challenge for the average non trained singer. As folk events started to use more amplification, the voices get quieter and more 'natural' as there is less strain on the voice, singers who did not sing in public before because they thought they were 'too quiet' now can use a microphone and be heard. Now we have a situation whereas (some) people who normally use mics just don't know how to sing without them. I see no point in singing a song in public (without aplification) where the audience just can't here the words being sung. So maybe the 'folk voice' wasn't so bad after all.