Oh my - lots can go wrong. Assuming 1) everything used to work, and 2) the amp and speakers are not the problem, then first, check all connections and wiggle/rotate in the jacks to get a good connection. Also realize that each app has a separate slider in the system mixer. Don't just look at the master slider - look at them all. Sometimes the slider responsible for the sound you want is not the app you think ! Also, my sound system has a physical master volume pot. It gets bumped by various nefarious activities on my physical desktop. If you have such a master pot, check that. Last thing I can think of is that games can and will alter the sound card settings. If you loaded a new game, it probably set the master volume on the sound card really low. Get the sound card app from the MFG and adjust as needed.
UPDATED: Additionally, check your source(s). If the source got wonky and wimpy, that could be it. A/B compare two sources. If one is strong and the other wimpy, there's your answer.
Good luck troubleshooting!
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