This is going to be a vague question, but I only watched this show sporadically. There was an episode in where sydney was out in a garden and it was really sunny and a song was playing in the background, it sounded like something from Peter Paul and Mary but I don't know. I think she said something to her sister about it being a song they used to listen to. If by some strange twist of events, someone remotely knows what I'm talking about, I would appreciate it.
shareHaven't got the answer but I know where you can listen to all of Peter Paul and Mary's songs until you find it. Go to Grooveshark.com and type the name of any artist or song into the search box. Dozens of songs will come up.
sharethere is a music guide for seasons 1-3 here; http://welcometoprovidence.tripod.com/pro/music.htm but if it was after that then sorry :S
Now wait just a clock tick...
I know you posted this question a long time ago and I'm not sure if you have found an answer, but I think this is the song you were talking about. It is called "Time Will Stay" by Jessica Owen. You can find it here:
http://www.myspace.com/jessicaowenmusic/music/songs/time-will-stay-20722556