Hello, you can set different tons for different types of message(Regular Messages, DING Messages, VIP Mesaages, Mentions, Red Packet Messages). Custom ringtones are not supported at the moment.
Mobile app: [Me] - [Settings] - [New Message Notification];
Desktop Client: right upper side [Profile Photo] - [Settings] - [Notice] - turn on/off [Message Tone].

[TIPS]
1. If you do not want to be prompted in a quiet occasion such as a meeting, it is recommended to change the phone to silent mode (please refer to your mobile phone mute mode changing method)；
2. If you have log-in on desktop client, you can go to DingTalk mobile app to set the [Desktop logged in, notifications off] at the [Message] interface.
Guess you want to learn more about：
How to set "Do Not Disturb"?
How to turn on the notification tones?
How to operate if can't receive message when DingTalk runs in backstage?