Community:Mute
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Mute, or mutism refers primarily to the absence of vocal communication on VRChat. "Muting" can also refer to the action of lowering or disabling the volume on a world, avatar, video player, or user. There are different reasons, as to why a user may be mute. Some users may also refer to themselves as a "mute". It is acceptable to let a user know that their microphone isn't on, but it may be considered rude to press someone on why they're muted.
By choice
This type of mutism is sometimes referred to as a personal mute. Users may wish to be mute by choice for several reasons. Reasons may include the desire to remain anonymous and not be identified by their voice, or to play or act out a character or persona, like a silent protagonist in a roleplaying video game. Users who are mute by choice may dedicate to relying on body language, emojis, and/or writing utensils to talk to others.
Medical reasons*
*Per VRChat's 
Community Guidelines: "Do not use VRChat as a medical or therapeutic tool, device, or methodology". Please consult with a licensed medical professional for questions or counsel; this Wiki is not legal or medical advice.
In speech pathology, aphonia is the body's inability to produce vocal sound. In rare cases, some people with social anxiety, or people on the autism spectrum (ASD) may withdraw from verbal communication or be unable to communicate, via 'selective mutism' that occurs from social triggers, being overwhelmed. Stuttering is another speech pathology that makes it difficult for the speaker to verbally convey their words.
Technical issues
General
On the Volume tab of the Quick Menu, the dropdown for Microphone will display settings for your audio output.
Make sure the Current Device is accurate for the microphone you want to use. If applicable, check to see if the external microphone is plugged in or connected, turned on, and not muted.
Back in the VRChat volume tab, under Mic Output Volume, make sure it is not at 0%. A good range for output volume is between 80% to 100%. Users may have difficulty hearing you if your output volume is lower. With Mic Activation Threshold, make certain that when speaking, the white line can go beyond the set line that indicates the percentage. An average threshold is between 5-10%. It is generally not recommended to have the threshold beyond 30% unless an external microphone is plugged in and very sensitive.
Lastly, make sure that Microphone Behavior is set to Toggle, or Push To Talk. You may also have your microphone set to Always On, but is not typically recommended.
PC / SteamVR
Make sure the Current Device is accurate for the microphone you want to use. If applicable, check to see if the external microphone is plugged in or connected, turned on, and not muted. Check recording devices in Windows by going to Sound settings or Sound mixer options, to make sure the correct microphone is set to default.
If you are connected to VRChat using SteamVR, open the SteamVR dashboard, and review your audio settings to make sure your recording device is set correctly.
Quest
When launching VRChat for the first time, make sure VRChat was given permission to let you use your microphone. If not, you can enable permissions by opening the Meta Quest dashboard, and navigating by clicking Settings > Apps > VRChat. You may also have to check the Meta Quest dashboard to see if your microphone is muted. The native Quest version of VRChat should detect your device's microphone by default.
Mobile
By default, your microphone is in "push to talk" (PTT) mode. You have to hold down the button to talk, and let go to stop talking. If the icon is white, no audio is detected. If the icon is red, it means audio is being detected, but you are not broadcasting your voice. If the icon is blue, audio is detected and you are broadcasting your voice.
When holding down the talk button, the audio in VRChat becomes quiet to prevent audio from leaking, or echoing from your broadcasted voice. If your voice is not getting picked up, you may need to go to the Apps Permissions section of your device's settings to enable microphone and audio. If connecting headphones to your devices, via jack or Bluetooth, check if those headphones have a built-in microphone and make sure it is enabled. By connecting headphones, you will also have the ability to set your microphone behavior to Toggle, or Always On.
Alternatives for Communication
Users who are mute can also communicate using the Chatbox to type their message, and display it over their avatar. This can be done by opening the Action Menu, by pressing 'R' on desktop, navigating to Options, or Tools, and using the Chatbox from there. If using a keyboard, a user can press the 'Y' hotkey to instantly open the Chatbox. Controllers can also be set to have a binding for the Chatbox.
Worlds may also feature pens, pencils, or markers to draw with. While limited, some mutes may utilize a list of Emojis to express their communication. Some mutes may solely rely on body language or answering 'yes or no' questions.
VRChat's hard-of-hearing, and deaf community developed sign language interpretation for expansive communication. VRChat also allows for a user to add their signed languages, or 'no lingustic content' to the languages displayed on their profile.
