OsmAnd is a navigation app, it is based on the OpenStreetMap (OSM) map database. The Data can be stored locally for offline use.
In the F-Droid store you get OsmAnd~, which is a version without GooglePlay services. As a there are no Google dependencies the voice output does not work because TextToSpeech (TTS) is not installed.
Through the [AuroraStore]() you can install the Speech Services (unfortunately) by Google. Then you can activate TTS in OsmAnd~ and use your navigation app.