Discord is free and the most popular team communication app for gamers. The platform provides users with familiar Slack-like UI, Skype-like video, and voice chats. Discord is amazing to use while participating in a Dota 2 battle, but it also meets collaboration challenges many companies face.
The tool divides channels by text and voice. ...
Mattermost is the main alternative to Slack that covers most of your requirements. I have not looked at it in a while, but you will also find a few useful others here in previous questions:
Team chat with private rooms, file upload and link previews (alternative to Slack)
Team chat with native Android/iOS apps, all open source (alternative to Slack)