ITS JUST A FIX , AM NOT RESPONSIBLE FOR ANY DAMAGE TO YOUR DEVICE , TAKE NANDROID BACKUP FIRST, DONT TAKE RISK.
Install rom in which your VOLTE WORKS, TAKE BACKUP OF MODEM , EFS, FIRMWARE (BOTH) PARTITIONS
Remember to backup 7.0 rom only ,
Then reboot your device, If you see VOLTE (HD) working,
Try to make a call, if call connects and there is no sound.
Just flash the MODEM and EFS partition from the backup you took
YOUR VOLTE WORK FOR SURE...