No worries as been kind of busy myself
1 No red or yellow X by audio.
2 The audio cd plays very faint for about 10 seconds at the beginning and then mutes for good.
Info from your OP + the above suggests that this is speaker related, either the speakers themselves are bad or the connection to the MB is bad/ not connected properly.
3 No speaker jacks that I can see.
Just watched a video of the same model of notebook as yours being unboxed and noted that there are no speaker ports, there is a headphone port so if possible try a set of headphones to see if you get sound through them,if you do, it again points towards either the speakers themselves being bad or the connection to the MB is bad/ not connected properly, this because the issue is present when using both the optical drive and youtube video, each are different sources of audio but both rely on the same speaker system for external sound to be heard.
4 No HDMI screen.
Acknowledged, pity as it would have helped with the trouble shooting, any family or friends that you could ask if necessary.