Hello Mark,
Thank you for using Apple Support Communities. Based on your post, FaceTime's audio is low even when the volume is up all the way. We'll do our best to assist.
It sounds like this may be helpful for you: If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker
If that doesn't sort it out, then test the hardware with the following:
- Set the ringer/silent switch to ring.
- Go to Settings > Accessibility > Audio/Visual
- Slide the audio volume balance slider all the way to the left.
- Slide the audio volume balance slider all the way to the right.
- Slide the audio volume balance slider back to the middle again.
If you noticed any sound issues with the left or right side, then you'd want to bring in the iPhone for an inspection: iPhone Repair and Service
Use this to be prepared for the visit: Get your iPhone, iPad, or iPod touch ready for service
Select your region and search nearby for locations: Find Locations
In case there isn't a sound issue on either side, then we'd recommend to review: If you can't make a FaceTime call on your iPhone, iPad, or iPod touch
Under section, 'If you have connection or sound quality issues with FaceTime'.
Also, see: Resolve Wi-Fi and Bluetooth issues caused by wireless interference
Should the issue continue, then it would be best to contact Apple directly: Get Support
Wishing you well, take care.