I currently try to develop a speaker that only implements the Alexa.Speaker interface. Discovery works fine and the device is displayed in the app. Then, I can successfully adjust the volume "Alexa, erhöhe Lautstärke von Lautsprecher um 20%" ( = "Alexa, increase volume of speaker by 20%") works fine. However, setting a specific volume does not work. "Alexa, setze Lautstärke von Lautsprecher auf 50%" ( = "Alexa, set volume of speaker to 50%") results in "Lautsprecher unterstützt das nicht" ( = "Speaker does not support this"). The function is not called at all, so I guess Alexa decides beforehand, that my device cannot handle the call and I have no idea how I can handle this issue.
In case it's relevant, I use Alexa in German.