My guess would be that you have one of the old mdp to HDMI adapters that do not support audio. What adapter do you have? Link would be good unless it's one of the adapters I have listed in post # 4.
Did you select headphones while the audio cable was attached?
You're using the same audio cable and both the HDMI and audio cable are plugged into the same ports on the TV you used with the old Mac?
I have recently been given an old Power Mac G4, 400MHz/1M Cache, DvD-V 64MB RAM, HD 20GB (I know low specs I mainly just wanna learn about macs) and I am trying to use my Sony Bravia KDL 26v4000 as monitor. The TV is 26 inches, and is 720p ... I have tried to use a VGA monitor cable to connect it all up, but when connected computer starts up for a bit, comes up with "Mac OS 9.1" etc.
Essentially though, you're using old technology, and that with limited specs, eg. the minimum RAM, as you've outlined. I don't enjoy saying it, but you're on a road to nowhere. You'll need to use much more modern and powerful hardware to achieve what you're trying to do.
1)
1. What Mac do you have? Personally, I prefer to have the year, model and processor speed. Example: 2008 iMac 2.8, 2006 Mac Mini 2.0, or 2009 MBP 2.6.
2. What TV do you have? This means make and model #. A link to your owners manual (not the spec sheet) would be helpful. But, if you can't find it, still provide your model # and we'll try to work from there.
Providing the ports you have on the TV will only allow for a "best guess", it would not allow us to provide a "definitive and specific" recommendation.
(If you just can't figure out what model you have, you may provide the name of every input port on the back of the TV and we can make a best guess. You should provide the name as they are labeled on the set or post a pic where those labels show up good enough to be read.
3. Where do you want the sound? On the TV, a stereo, a 5.1 A/V receiver? If you want it someplace other than the TV, you'll need to provide the make and model of that piece of equipment also.