OK, got it to work now. 
To get the BOOTLEG devices I removed the Alexa voice assistant in the Sonos app, removed the devices in the Alexa app, then re-added the Alexa voice assistant in the Sonos app and with that got everything back in the Alex app. Now for all Sonos devices that have Alexa enabled I also got two devices in the Amazon Echo Control page, one with family “THIRD_PARTY_AVS_MEDIA_DISPLAY” and another with “THIRD_PARTY_AVS_SONOS_BOOTLEG”.
And for the Things created with the serial number of the “THIRD_PARTY_AVS_SONOS_BOOTLEG” device, the speak channel now works. In my case this are Sonos Ones (2nd generation) and a Sonos Beam. @ML1982, have you in the meantime been able to get it to work on your Ones?
What does not work is use Sonos devices that do not support Alexa because of the lack of microphone, such as the Amp or One SL. For those there is no “THIRD_PARTY_AVS_SONOS_BOOTLEG” device with the ALEXA_VOICE capability that seems to be needed for the speak channel to work.
Another small issue I have found is that it seems the volume channel of the Echo thing does not get updated if I change the volume via the Sonos binding or the Sonos app. But when using the speak channel while playing music, the volume of the music then gets set to the value the volume channel of the Echo Thing, not the actual volume that was playing via Sonos.
This can be remedied by setting the Echo volume to the Sonos volume before issuing the voice command:
items.getItem( "Echo_Control_Sonos_Badezimmer_Volume" ).sendCommand( items.getItem( "Sonos_Badezimmer_Volume" ).state );
items.getItem( "Echo_Control_Sonos_Badezimmer_Speak" ).sendCommand( '<speak>Hello World!</speak>' );
And I have another issue/question that may be a bit off-topic because it is probably not related to Sonos. I’d like to use the speak channel to play a notification sound via the <audio> tag. However that does seem to work only in conjunction with text.
This works, with spoken text:
<speak>Now comes the sound: <audio src="soundbank://soundlibrary/home/amzn_sfx_doorbell_chime_01"/></speak>
Without spoken text it does not work:
<speak><audio src="soundbank://soundlibrary/home/amzn_sfx_doorbell_chime_01"/></speak>
Does anyone has an idea how to make a notification sound work without spoken text?
Why I want to do that? There is a notification sound channel in the Sonos binding that could do this as well, but with that, any music played from Spotify Connect is stopped and not restarted after the notification. When using the speak channel via the Echo binding that is not an issue, music continues to play.