The general rule:
The admin of a group can delete all voice messages at any time.
The sender of the voice message can delete their own voice messages at any time.

Voice messages can be deleted in Sdui in two ways:

Delete in chat:
If you want to delete a voice message in the chat, click (on the PC) on the three dots at the top right of the voice message. Hold your finger on your message for a long time on your cell phone. Then click on "delete".
After deleting the voice message in the chat, it is also deleted in the "Voice Memos" folder.

Delete in the cloud:
Go to the cloud folder of the chat where you want to delete the voice message. Then click the Voice Memos folder and locate the memo you want to delete. Click on the three dots to the right of the file name and then click on "delete".
The voice message is also deleted in the chat after being deleted in the cloud.