I’m using Audacity 2.1.0 on Windows 10 64-bit, which has worked flawlessly for as long as I can remember. But as of today, playing OGG audio in VLC on Windows, or dropping MP3 files and playing them in Audacity, adds a reverb quality to the sound as if it’s coming through a crappy phone speaker. This effect doesn’t occur with the same MP3 files before dropping them into Audacity and didn’t occur with the OGG files beforehand. I restarted Windows and downloaded OGG codecs for Windows from Xiph.org but the problem persisted.
But as of today, playing OGG audio in VLC on Windows, or dropping MP3 files and playing them in Audacity, adds a reverb quality to the sound as if it’s coming through a crappy phone speaker.
It could be [u]Windows Audio Enhancements[/u] or your computer/soundcard may have a utility for adding effects.
This effect doesn’t occur with the same MP3 files before dropping them into Audacity and didn’t occur with the OGG files beforehand.
Simply opening/reading a file into Audacity doesn’t alter the file. If can only be altered if you export to a new file or if you export and overwrite the original file.
It could be Windows Audio Enhancements or your computer/soundcard may have a utility for adding effects.
I checked Realtek HD Audio Manager out and it doesn’t have any alterations in effect. Besides, how could an alteration affect one audio format but not others?
You told us that “Ogg in VLC” and “MP3 in Audacity” have the problem. That’s not “one audio format”. 
DVDdoug is correct that Audacity does not modify the sound unless you tell it to (by applying effects), and then you need to Export the modified audio to create a file that has the effect applied.
I can’t reproduce the problem that you are describing. Can you give step by step instructions for how you can reproduce the problem (remember that we can’t see your computer, so we only know what you tell us).
It had slipped past me, but when I retested with sound channels, it turned out that the reverb was caused by running 5 channels instead of 2 in Realtek HD Manager. Thanks for your replies, gentlemen!