Contacts* not contracts ;P

I'm pretty sure you can... The same way you would customize an individual contact. Long hold the group conversation> upper right corner choose "Override Default Settings"> Notifications> Notification Sound> Choose your desired tone. I just tried it to make sure changing a groups conversation settings doesn't change the individual customized settings of the contacts inside the conversation and it didn't. The contracts settings stayed the same while anyone replying from inside the group conversation will function based off default or your customized settings. GOOD LUCK!