Youtube + Airport Express

Whether you use iTunes or a web browser, AirPlay streaming requires a QuickTime CODEC to convert audio to the Apple Lossless format before it can send it to the Express. A dropped stream indicates an issue between these two points, typically that the communications link cannot be maintained.

AirPlay basically does the following:

• Locate an active AirPlay "speaker" on the local network.
• Establish a secure connection with that speaker.
• Start and maintain the stream.
• Periodically, verify that the connection is "good."

It appears that the last item fails and the stream is terminated in your case. Which would require further troubleshooting will be necessary to find out the culprit(s).

If you wish to pursue this course, then we would need to know more about your network configuration with your AirPort Express.

1. Is the Express the only router in your current network configuration. If it is, what is the make & model of the Internet modem that it is directly connected to by Ethernet? If it is not, what are the make(s) & model(s) of the other networking hardware?
2. Where is your notebook located, in reference to the Express, when you attempt to stream? Same room, different room, or different floor?
3. If there is more than one router, where is this other router located in reference to the Express?

So YouTube it not a factor at all ? If all audio airplayed from Your computer suffer from this and not just YouTube then we can rule out YouTube as being related or relevant to your issue

If the image you provided, your AirPort Express is being recognized by your Mac, and any audio from your Mac should be sent to this base station. Are you saying that all audio, expect YouTube (via Safari) does not play. How about another Internet audio source from the Safari browser or by using another browser?

what browser ? because on safari on my mac only by appletv show up as airplay devices

where in iTunes both expresses does too