Comment by hnlmorg

For Slack, I find just changing the default notification sound to a simple and subtle ding works well.

When I’m focused, I don’t hear it because it’s too subtle. But when I’m not concentrating on anything, it’s more noticeable and I don’t mind the distraction.

This might not work for everyone (“YMMV” and all), but I’ve personally found it a very effective yet simple solution.