When playing iTunes from 2006 MacBook via Airport Express remote stereo speakers, the music keeps dropping in and out

Question

I've tried hours and a trip to the Apple Store to resolve this problem, and no luck so far.
We have two Airport Expresses -- one is an older "g" version (from 2006), one is a newer "n" version (a couple of years old).
We have one older MacBook (from June 2006) (so it can only use "g" wireless and also cannot be upgraded to OS X 10.7 "Lion").
We have one MacBook Air and an iMac (both less than a year old), an iPad (about 2 months old), and a recently acquired AppleTV (in the past few weeks).
We also have two iPhones that do use our local wireless network on occassion.
We have stereo speakers that are currently connected into one of the Airport Expresses.
For years (until the past few weeks) I played music from iTunes on my first generation MacBook to the speakers with no problems (using the older "g" Airport Express).  Recently, however, when playing music remotely over the speakers we have gotten fairly regular (though at random intervals) drops of the music in and out (drops are for a second or two each).  So far, here's what we've tried to do:
(1)     Went to the Apple Store, where they thought the problem might be compatability between the g and n equipment in our house.  They suggested that we get an Airport Extreme and use that as the base so that 2.5 GHz and 5 GHz networks could each be set up, allowing the two systems to play nicely.  Spent a couple of hours setting everything up.  No dice.
(2)     The skipping happens even when (a) I turn off the Apple TV; and/or (b) turn off the older Airport Express and just use the Extreme and the "n" Express (with the "n" Express as the connection to the stereo).
(3)      When I have the wireless network drop-down menu selected on my MacBook, the skipping occurs immediately after the computer is "looking for networks" (as shown at the top of the menu).  The computer does this at random intervals on a regular basis.  The skipping, however, will occur even if the menu is not selected.
(4)     Here's the weirdest part.  The skipping has even occured when playing iTunes from our newish iMac, with the older MacBook shut down, with the older Airport Express shut down.  In this case, the skipping only appears to occur when we have the wireless network drop-down menu selected and again, it occurs when the computer is "looking for networks" which occurs at random intervals.  In this situation, the skipping only seems to occur when the menu is selected.
At this point, if I can't find another solution, I'm going to return the Airport Extreme, and just give up playing music over our stereo speakers from the MacBook (and instead transfer music to the iMac and play from it).  But I thought I'd try a last-ditch effort to ask for help to see if anyone could figure out what might be causing the problem.  I would chalk it up to the problems of using my old "g" network MacBook on a newer "n" system (though I'm not about to spend $1K+ to upgrade my laptop just for this), but for (4) above, which seems to indicate something else is going on.
Any help would be most appreciated!

Answer 1

Unfortunately, your description of the problem and the placement of both your main and AirPort Express routers does lend itself to believe the problem is most likely Wi-Fi interference. If it isn't the microwave or portable phones, it could be obstacles like walls between the routers that may be preventing the stream from consistently reaching the Express.
In your configuration, the stream will first travel to the main router, and then, from the main router to the AirPort Express. If, for example, you started the stream in the same room as the Express, that stream would have to travel across several rooms at least twice.