My initial thought for an investigation was to analyse the way a teenager speaks with a person of a similar age compared to a conversation with an adult.
The way I could do this is record my 15 year old brother speaking with his friend, and then speaking with our mum. I think this would be interesting as it shows the way he may change his intonation when talking (speaking more deeply with his friend), changing the style he speaks (pace and phrasing) and the words he uses and how (diverging from slang/swear words and changes in his accent).
I find this interesting because I already notice the changes in the way he talks and I know this is a common denominator amongst most teenagers.
I did think of investigating teenagers speaking and then a group of adults but I think it would be more interesting by just using the same person in two different contexts as it clearly shows the changes that automatically appear. :))