AirPlay Music From AirPort Express Dropping Out
I have struggled with a similar issue for about a year now. Under the veil of secrecy, I think that airplay is a solid protocol.

I use an AT&amp;T Arris Gateway for a DHCP server with an Airport Express as a LAN node. Here's what seems to have solved my problem:

Deep inside the configuration pages for my Arris router (Firewall > Advanced) I found a "Flood Limit" option, which was stated to block "bursting new traffic." Essentially, the router was dropping packets after a certain limit had been reached (I believe this is a mechanism against DoS attacks). Disabling this seemed to stop the AirPlay from dropping out.

Long story short, you may have a router that is tampering too much with the packets on your LAN. Go through every setting on your routers/APs. Disable MAC address filtering, make sure these TCP/UDP ports are open, and steer clear of firewall features that are degrading the performance of your network. Also, wired connections between APs really help with streaming.