how can I get Siri to give audible directions when driving?

Provided that your region has the features outlined by SravanKrA above you can follow the instructions in Apple's support document Get driving directions in Maps on iPhone.

You can fast-forward to the list of countries using turn by turn navigation here.

I activate it simply by asking Siri to find a route to an address, touch on the estimated journey time, and then press the Go button on one of the routes offered as options.

iOS and iPadOS - Feature Availability - Apple

Choose a feature below to see if it's supported in your region and language. Maps. Standard; Satellite; Directions; Turn-by-Turn Navigation ...