Now got a relatively satisfactory solution via the Amazon Echo Control binding. Those Sonos devices that have Alexa enabled as voice assistant can also be controlled as Echo devices via that binding. There is a speak channel that allows for Alexa announcements, which can also include audio files similar to the notification channel from the Sonos binding.
There are still some limitations. The most obvious, it does not work on Sonos devices that do not support Alexa, like the Sonos Amp. In our case we have Echo Dots paired with the Amps, so for those rooms I just use the Echo Dot for the notification.
And right now I cannot get Alexa to just play an audio file, it always needs to come with a spoken text. Still trying to figure that one out.
Last but not least there was an issue with the volume set after playing the notification, but that could be solved by bridging the volume from the Sonos binding into the Echo binding.
See also this thread/post:
@Lolodomo: not sure if that is possible, but maybe the Sonos binding also has access to the same functionality the Echo binding? But of course it could well be that that this works only via the Amazon Alexa cloud.