[robotmay]

Good question! I'm not super knowledgeable about it myself. I think anything from that far back is likely to be a reasonable amount of guesswork, but there are often some indicators as to how things are pronounced. A good one I've seen before is looking at rhymes; if it's likely that two words were intended to rhyme in a song/poem etc it can help indicate some pronunciation. I've seen this used for Shakespeare original pronunciation studies which is quite interesting.

From personal experience of trying to play old tunes (rather than songs), I definitely have to make my own interpretation of it based on my own experience. Sometimes you do come across some truly perplexing notation though, that really makes you wonder how it sounded originally. Some of the tunes in the Playford collection are super weird but as Playford was basically Top of the 17th Century Pops you have to assume it _did_ sound good however they used to play it :D