Something to note, only certain Alexa enabled devices support being used as Bluetooth speaker. I would recommend checking with Amazon to determine the capabilities of your specific speaker model. If you have the ability to connect via Bluetooth from your computer settings, you should be able to connect to your speakers.
However, if you’d like to cast from your computer to an Alexa enabled device, this is only a feature of Amazon at this time. Additionally, while using your speaker via Bluetooth with your computer - your Alexa commands may not work.
For additional help with connecting your Amazon Alexa device, I would recommend checking here for more details.