Apple has released AirPort Base Station and Time Capsule Firmware Update 7.6 which is intended for all 802.11n AirPort Express, 802.11n AirPort Extreme and Time Capsule models and provides fixes for the following:
• An issue with performance on overlapping wireless networks
• An issue with AirPlay audio streaming
• An issue with configuring multiple entries for DNS servers
Detailed information on Firmware Updates 7.6, including how to install the update here.