I have sorted it, anyone else with the same problem needs to do the following:

sudo nano /boot/config.txt
And scroll down to the bottom. If you see a line that says: dtparam=audio=on

Disable it by putting a # in front.
Then add:
dtoverlay=hifiberry-dacplus
on the next line. Save the file.

Next create /etc/asound.conf with the following content:

pcm.!default {
type hw card 0
}
ctl.!default {
type hw card 0
}

Then go to: /home/pi/.reicast/emulators.cfg and change it to this:

reicast-audio-alsa = "CON:/opt/retropie/emulators/reicast/bin/reicast.sh alsa %ROM%"
default = "reicast-audio-alsa"
reicast-audio-oss = "CON:/opt/retropie/emulators/reicast/bin/reicast.sh oss %ROM%"

Save and reboot and enjoy!

Also has anyone managed to get reicast to change to 16:9 aspect ratios? maybe i'll start a new topic for this.