I also have a Tour...and love it. The Mute key is located on the top of the device. If you are looking at the screen, look at the top right. If you press there, it wil mute the phone if you are in a call. If you are not, it will place the phone is a standby mode until you press the mute button again. The top left key is the keypad lock key. On the Tour, Blackberry did not make the keys as noticable as on the other devices such as the Curve or Pearl. I kind of like it because it keeps the phone looking sleek. I hope I helped you.
You are a life saver as I was going nuts as the phone was telling me to press the mute button and I could not locate it, did not have the book home and the website I was not finding it on the website.
Thanks and have a great day.