On the first partition of your SD card, you will find a file named config.txt. This file allows you to tweak a lot of things like framebuffer resolution, overclocking, audio, and is documented here

Some TVs send a wrong EDID saying that HDMI audio is not supported while in fact it is. You may need to force HDMI audio by uncommenting this line in the config.txt:

hdmi_drive=2

If your RPi is not booting while using an HDMI to DVI adaptor, please remove this line from your config.txt:

hdmi_drive=2

If you are using a HDMI to VGA adaptor on a Raspberry Pi 1 or 2 and you want to use the jack output for the sound you have to write in config.txt on the first partition of the SD card:

hdmi_ignore_edid_audio=1

So the board'll not use the HDMI sound output.

Since Lakka version 3.5.1 it is possible to use CRT output. Here are examples for 240p mode:

Add video=Composite-1:720x480@60e to cmdline.txt right after quiet (separate these statements with a space).

Staring from Lakka 4.0 add following line to config.txt:

dtoverlay=vc4-kms-v3d,composite=1

Add video=Composite-1:720x480@60e to cmdline.txt right after quiet (separate these statements with a space).

Add following lines to config.txt:

enable_tvout=1
dtoverlay=vc4-kms-v3d-pi4,composite=1

If you are missing the audio output device for the 3.5 mm jack, following lines have to be added to config.txt to enable audio output via the 3.5 mm jack:

dtoverlay=piaudio
dtparam=audio=on

These lines must be in this order and no other config lines should be between them.

In RetroArch set the audio output device to default:CARD=Headphones to route the audio output to the 3.5 mm jack.