It sounds like you're having some trouble hearing Siri give voice feedback when you're not using your headphones. I like having Siri set up to meet my needs, and I want to help you get Siri configured to your liking.
Let's go to Settings > Siri & Search > Voice Feedback. Here we need to make sure that the Always On option is checked.
After you have Always On checked, go back to the Home screen and test invoking Siri without your headphones in.
Is this working as you expect it to now? Let me know if this helped.
Here are some other useful tips for Siri & Search:
Use Siri on your iPhone, iPad, or iPod touch - Apple Support
Make requests - iPhone User Guide
Thanks for using the Apple Support Communities.