Having heard many recordings of old time music hall acts, I wonder if their delivery in terms of pronunciation etc could also be due to trying to be loud enough to be heard?

With microphones, many people speak and perhaps even sing differently to when they had to belt it out. I know that when I have played even folk clubs in very large rooms, my talking is slightly different to if I was either in a smaller venue or mic'd up in a theatre. I also find myself singing slightly differently if I have to be louder.

The commonality of music hall accents could be in order to be heard and understood. A music hall with 500 noisy punters makes you think about being understood and your regional accent may well be a casualty....