I think your issue might be trying to split the mic output to two sources.
1. Powered speakers
2. Non powered headphones.
The audio card may not provide a strong enough signal.
If your speakers are connected to this Y cable configuration the you must only get sound from the mic through them and no sound from the computer itself. Or am I still missing something. I don’t know why you would want speakers to work at the same time you are wearing headphones.