2. X-plore or any other file manager with root access
1. Extract and replace "mixer_paths.xml" in root/system/etc
2. Change the permissions to rw-r--r--
3. Restart your mobile
I won't be responsible if anything goes wrong.
I'm also providing the stock xml file if you want to revert back to the original settings.
Thanks to AndroPlus for providing custom kernel & recovery for the Z5P Dual.
***Version 2 added with sound from both speakers in speaker mode during a voice call***