You can increase or decrease a contact's volume by one of two ways:
1. On talk screen, slide finger from outside to inside of talk button to turn on volume indicator. The ring will turn blue and a "dB" number will display. Run you finger along the ring to increase or decrease the dB setting.
2. In the contact's profile, use the dB slider to adjust volume.
The change will only affect the selected contact and not other contacts you may have.