Hi, iPhone6,
Thanks for visiting the Apple Support Communities. If I understand, you're trying to pair a Bose Soundlink Mini speaker with your iPhone but don't get any audio from the speaker once you do. I'm happy to help!
First, go through the steps outlined in the Apple Support article If you can't connect a Bluetooth accessory to your iPhone, iPad, or iPod touch - Apple Support. Even though the article is specific to issues connecting, the steps included can also help other issues with Bluetooth, such as what you're experiencing.
If those steps do not help, please refer to the Bose troubleshooting steps here: No audio from a Bluetooth device. For additional assistance, please get in contact with Bose directly to confirm your speaker's compatibility with iOS 12, as well as to see if there are any firmware updates available for your speaker: Contact Us.
If you still need help after that, you can reach out to Apple Support directly: Contact Apple Support.
Have a wonderful weekend.