Hi Lori, we took the tour last July, and were in Murren around the middle of the month. Unfortunately we were stuck with pretty much lousy weather all 3 nights, with only a few brief periods of sun or partly cloudy skies. I would say the daytime temps were about in the 40s or low 50s; rain was common and frequent. Thing is, the Alps make their own weather so it could be totally sunny in the morning then change a few hours later.
You'll definitely want a sweater and a lightweight, waterproof jacket for Murren, plus an umbrella. My wife dressed in layers. It was obviously more chilly walking around in the early morning or late at night. I also packed a beanie.
If weather is bad on your free day, your guide may well take you into Interlaken as an option. It may be rainy, but not terribly cold. If you're hiking or going higher up in elevation, you'll obviously want to dress accordingly for that. Trummelbach Falls, regardless of the weather, is basically inside a cave and its very wet, so you'll want to dress warm and in a jacket in there.
We just posted our scrapbook. It's got more indepth info on our time in Murren plus packing lists, but let me know if you have any other questions. It's a great tour, you'll love it!