I tried about 4 to 5 different volume apps, equaliser apps etc. but still didn't solve the issue.
I called the HTC Service dept. (in Singapore) and was told that this is the standard features of HTC One that the In-call volume will always return to 3/4 setting whenever the phone is switch on. They told me that others have also brought up this issue but there is no way to adjust or set the volume to a desired setting, unless you don't switch off the phone.
Looks like we have to accept this small inconvenient for this excellent phone. Life is never perfect.
Hopefully the coming 4.2.2 upgrade is able to addresses this issue.