@Rcs we have no clue what version of retropie, raspberry pi (also what version of pi) or pc or any of the other stuff that readme says you need to give us info on. If you give us something to start with then maybe someone can help. I would start by going into the file /boot/config.txt and removing the hashtag from the beginning of this line:
#hdmi_drive=2
So it looks like this:
hdmi_drive=2
Then save and exit and then reboot. If you are using a raspberry pi and you dont ftp into your pi then plug in a keyboard and hit f4 to get to the command line. Now type:
Sudo nano /boot/config.txt
Now scroll down and until you find the line that says:
#hdmi_drive=2
Then remove the hashtag so it looks like this:
hdmi_drive=2
Then save and exit by hitting "ctrl & x"at the same time then "y" to save. Then type
sudo reboot
in the command line. Also when in emulationstation hit your start button on your controller, go into sounds and change the volume from 77% to 100%.
If that doesnt work then you need to give us more info on your setup just like that "read this first" link tells you what all info we need.