lint: fix linter warnings reported by golangci-lint (#522)

- Fix #519

audio_test.go

@@ -40,7 +40,7 @@ func TestAudioWithFailingFormBuilder(t *testing.T) {
 	}
 
 	var failForField string
-	mockBuilder.mockWriteField = func(fieldname, value string) error {
+	mockBuilder.mockWriteField = func(fieldname, _ string) error {
 		if fieldname == failForField {
 			return mockFailedErr
 		}