Fix: get audioBitrate instead of audioRate (#14)

* fix: get audioBitrate instead of audioRate

* fix: fix the sprintf AudioBitrate as well

Author: xage93

output_getters.go

@@ -97,8 +97,8 @@ func (a *Args) GetAudioRate() []string {
 
 // GetAudioBitrate returns the arguments for bitrate
 func (a *Args) GetAudioBitrate() []string {
-	if a.output.audioRate != 0 {
-		return []string{"-b:a", fmt.Sprintf("%d", a.output.audioRate)}
+	if a.output.audioBitrate != 0 {
+		return []string{"-b:a", fmt.Sprintf("%d", a.output.audioBitrate)}
 	}
 	return nil
 }