My video card is a Radeon R7 360 based on the same chip as the 260, 7790 ...
My audio was with noise and distortions by the hdmi.
The solution I found was:
1) log in with ssh
2) type in the terminal
# aplay -l
* List of PLAYBACK Hardware Devices *
card 1: HDMI [HDA ATI HDMI], device 3: HDMI 0 [HDMI 0]
Subdevice # 0: subdevice # 0
The list was much bigger. I just pasted the relevant information.
3) and then edit the file /recalbos/share/system/.asoundrc
with the information that matters, CARD and DEVICE, in my case CARD 1 and DEVICE 3.
I deleted all the content and pasted only the lines below:
4) I also edited the file /recalbox/share/system/custom.sh
for every time you boot the system, it rewrites the .asoundrc file with this setting.
echo "defaults.pcm.card 1
defaults.ctl.card 1 " > /recalbox/share/system/.asoundrc
and then on the terminal:
#chmod + x /recalbox/share/system/custom.sh
I hope it helps.
Sorry for the bad english.