When it's off, Zoom will automatically manipulate your outgoing audio with noise suppression, will eliminate unwanted low frequencies, and adjust the volume of sounds. Original sound being off is ideal for regular office meetings or for any noisy environment. Enabling it is only recommended for playing music or for environments where studio-level equipment is being used.
Keep in mind that when you turn it on, some other audio options will still be enabled by default, such as Echo cancellation and Stereo Audio. A separate option called Suppress background noise may also still be enabled (it can be found in the Audio tab).
It is possible that due to these other settings you (or better said your listeners) are not noticing much of a change, or that you are in a low-noise environment so it's not as noticeable. Also, keep in mind some devices/headsets have some of these options built-in and would have to be disabled on the device level if possible.
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