If you installed the latest recommended Forge source (.80), the only thing you need to do is copy the .minecraft/assets/ folder from your working 1.7.10 install into eclipse/assets folder. Program arguments should be EMPTY.
This is done intentionally, it's not a bug - the sound data is another 100MB and only the official Minecraft launcher can download it (for now, Forge don't see the need in putting high priority on it when a dev can fix in 1 minute and there is much more important work elsewhere for Forge).
Worked fine for me, I did it just a few hours ago. You did something wrong. Make sure your program arguments are EMPTY, and that your eclipse/assets/ folder matches the same as your .minecraft/assets/ folder. You don't have an eclipse/assets/assets/ folder do you? Also you have to actually have 1.7.10 installed and working on your PC already for that to work, because it downloads sound to that folder on first run.
Dunno what else to suggest, it works fine. Simple logic, just make your Forge instance match a real instance regarding it's assets.