I release final solution for the bluetooth calls echo issue. Solution is for the PX5 units. For now it works only with MD725 module.
If you want to use this solution in your custom ROM, please put my "Donate to me" button in a noticeable place.
-Added patched version of sdsdk
-Fixed problem with auto connect after sleep
-Added libgnustl_shared.so to zip archive because not all ROMs have it.
1. Copy gocsdk, sdsdk, libf1xaec.so, libgnustl_shared.so and f1xaec.ini files to your SD card
2. Install a file manager (like ES File Explorer) which supports exploration of Android system dirs.
3. Go to /system/bin directory
4. Find gocsdk file and rename it to gocsdk_backup
5. Copy gocsdk file from your SD card to /system/bin
6. Set permissions (access rights) to 777 for /system/bin/gocsdk
7. Find sdsdk file and rename it to sdsdk_backup
8. Copy sdsdk file from your SD card to /system/bin
9. Set permissions (access rights) to 777 for /system/bin/sdsdk
10. Copy libf1xaec.so to /system/lib directory
11. Set permissions (access rights) to 777 for /system/lib/libf1xaec.so
12. Copy libgnustl_shared.so to /system/lib directory
13. Set permissions (access rights) to 777 for /system/lib/libgnustl_shared.so
14. Copy f1xaec.ini to /system/etc directory
15. Set permissions (access rights) to 777 for /system/etc/f1xaec.ini
Now it is very important to figure out what binary is used by your Head Unit. It can be either sdsdk or gocsdk. Right now rules of choosing binary by a Head Unit are unknown - probably it depends on MCU.
16. Install a terminal emulator (like Termux)
17. Execute command ps | grep gocsdk and ps | grep sdsdk
18. Remember what process is running (gocsdk or sdsdk) (if execution one of these commands gave you more lines of output, it means that this binary is used by the Head Unit)
19. Open /system/etc/f1xaec.ini
20. If your Head Unit is using sdsdk, change value of the streamDelay parameter to 280
21. If your Head Unit is using gocsdk, change value of the streamDelay parameter to 210
22. Restart Head Unit.
Short info: libf1xaec.so is a shared library containing implementation of webrtc algorithms with my patches and adjustments for RK3688 SoC. Do not worry - any viruses or other spyware are not included .