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I'm having the same issue. I tried repairing and that didn't seem to help. I have not made an exhaustive list of those sounds I can't hear, but I know I can't hear abilities/skills or jokes/taunts, and I can hear the announcer, random quotes caused by moving champ, and shop sounds. However I think I may know what the root cause is. Hopefully a red see's this and is taking notes.
If my champ is on the screen and I use CV so that my champ is within the vision range granted by it, then I can hear my champ again. I did not think to check if you actually had to be standing in the effect of the skill or just be within visual range of it for this to work. But I did check brush. If you are standing in the brush and cast CV your abilities will not make a sound, but if you move out of the brush in time for the CV to grant sight on you, it does. After you have been "spotted" your abilities will continue to make sound until you scroll the camera away from your champ, after which you will become mute again.
Hopefully this helps solve this issue sooner, rather than later. I don't imagine I'll be able to do any serious playing until this is fixed. I rely too heavily on the sound of approaching enemies for this not to effect my gameplay.
Edit: Just realized I necroed a thread... Thought that was today's date for some reason. Regardless. This is still a problem I'm currently experiencing.