There are several different ways of doing this depending on several factors not revealed in your question. The methods include:
HOUSE FEED
This is a mic-level or line-level signal from the venue sound system. Many more modern facilities have a “Press Feed”
connector available for visiting videographers (including news reporters, etc.) Or. If the event is using a contracted sound system, the system operator may be able to give you an audio feed. The advantage of this system is that typically ALL of the microphones will be covered in a multi-microphone situation. Disadvantages include that this is not always available, it requires prior arrangement with the sound contractor, etc. This method requires coming prepared with a modest variety of different connectors, adapters, cables, isolation transformer, etc. But if you are doing many events, this kit is worth it and it is not large or expensive.
MIC SPLITTER
The event operation may permit you to insert a microphone splitter gadget between the microphone and the venue sound system. This is a small, modestly priced gadget that will allow the venue sound to pass through normally, but gives you an isolated “split” of the audio that you can connect to your camcorder or recorder.
PARALLEL MIC
You can just put up your own microphone next to the venue microphone. An extreme case of this can often be seen in press conferences on the TV news, etc.
ACOUSTIC PICKUP
You can put your microphone close to a venue PA speaker to get a reasonably good pickup of the audio. Get the microphone as close to the speaker as possible. If it is more than a few feet away, you will start getting ambient noise and reverberation which is probably undesirable
All of these methods have advantages and disadvantages. Many (most?) of them require some sort of prior arrangement with the venue or sound contractor. In a large, complex event perhaps weeks ahead of time, or in a casual setting perhaps something arranged by showing up an hour early and buying the sound guy a soda, etc. And there are many variations like wired or wireless (if you can’t or don’t want to stretch wires across the floor, etc. And like running the sound directly into your camcorder vs. using a small handy-recorder at the sound board recording the event. You can then drop the sound track into your editor and sync it with the video, during editing, etc.
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Recording audio without metering and monitoring is exactly like framing and focusing without looking at the viewfinder.