sometimes when an antenna is used to pull in a signal.. other signals can come along with it.
maybe those other signals are enough to provide an ambient field that is enough to keep the speaker working.
would your metal roof have anything to do with it?
it isnt impossible that there is some reflection going on, but if there was interference.. then the interference is going to cause a problem regardless.
the metal roof probably isnt amplifying the signal (but stranger things have happened)
i would think the roof is simply acting like a mirror, and pointing the light towards some place that allows for an accumulation of radio frequency.
here is a simple example you could try yourself.
get a flashlight and a mirror.
point the flashlight at the mirror and point the mirror at the wall in a dark room.
you will see how bright everything is with the mirror pointed at the wall.
then move the mirror to the corner of the room and watch the brightness increase.
usually you have to be 3-5ft away from the wall to get a good look.
but if your roof is taking the radio frequencies and pointing them into some type of corner, then the extra brightness might be what is causing the wireless speaker to cut out.
if that isnt true, then the wireless speaker is probably junk.
maybe there is some intermittent interference coming from some place.
is there a pattern to the sound cutting out?
kinda hard to believe that the thing works most of the time and cuts out.
but that is usually everything digital modulation is about.
it sends out a varying signal of different amplitudes/energy levels.
and when it comes time for the weak signal to be read, there isnt enough strength.
old analog wireless was all about a constant electrical strength.. and if you werent close enough, you had to increase the power.
this helped people learn how to narrow the signal and point it.
narrow the receiver's sensitivity and point it.
same techniques have stemmed growth to today's digital modulation and pointing/shaping the signal.
if i was you.. i would measure the distance from speaker to transmitter and try the speaker someplace else with the same distance.
humidity might make things less or worse (as the signals get better at night)
i would think that in some ways the metal roof would shield the wireless speakers . but if nothing is obvious about the source of the problem , you will want to try to make a connection or rule out what you can . if you can't link anything else to this problem i would guess something defective with your wireless speaker setup .i have some wireless headphones , they have a little age to them , they still have a dial on each part and i have to sync them or they sound fuzzy or distorted and somethings cause them interference , like a kitchen mixer , or lawnmower , and on occasion they have intermittently encountered noise briefly .