Most Mumble audio problems are on the player's side. Start there, then check the server.
Run the Audio Wizard
In the Mumble app open Configure, then Audio Wizard. It walks you through choosing your microphone and speakers, setting the input volume and picking a sensible voice-detection level. Run it again whenever you change headset.
Choose how you speak
In Configure, Settings, Audio Input:
Push-to-talk sends sound only while you hold a key. It is the cleanest for noisy rooms and for gaming.
Voice activity opens the microphone when you speak. Set the thresholds in the wizard so breathing and keyboard noise do not trigger it.
Quality and delay
In Audio Input pick the Quality preset. Higher quality uses more bandwidth. If a connection is poor, a lower preset and a longer audio per packet setting reduce the dropouts but add delay.
Fix echo and noise
Use headphones. Turn on Echo cancellation only if you must use speakers. Enable noise suppression in the same page.
The server's limit
The server caps how much audio each person may send, so a user cannot flood it. If everyone sounds low quality, that cap may be set low. You can change bandwidth in murmur.ini under Settings, Config files, then restart. The default suits voice; raising it only helps if players also raised their quality setting.
Choppy or robotic sound
It is usually the player's connection: Wi-Fi, a VPN or heavy downloads. Try a cable or pause downloads.