Add volume changing logic

internal/mixer/mixer.go

@@ -247,6 +247,16 @@ func SetAllSinkVolumes(level float64) error {
 	return nil
 }
 
+// ConvertVolume converts a desired volume percentage (0%, 25%, 33%, 90%, etc) to
+// the float64 value needed to pass to SetAllSinkVolumes 0..1.0
+// Percentages should be passed as integers. 25% -> 25
+func ConvertVolume(volPerc int) float64 {
+	if volPerc == 0 {
+		return 0.0
+	}
+	return float64(volPerc) / 100.0
+}
+
 // parseSinks parses the output of `wpctl status` and extracts audio sinks.
 func parseSinks(data []byte) ([]Sink, error) {
 	var sinks []Sink