You're a god.
Jaffa, kree!
I put a 6.8k resistor inline, and now DREQ still works, and powerdown mode only consumes 2.2mA.
Interesting and strange at the same time, but good that it works now.
Does the Spark-IO really go low in the sleep mode?
Maybe, if it really was a short, there could allready have been a tiny bit of magic blue smoke leakage.
I'm still not sure why it's not in the uA range, but that may just be the SD card- not sure.
Yes, that is the sd-card, they have horrible stand by currents.
You are lucky that is only draws 2.2mA, that could be much, much worse.
You have 4 components consuming power:
Regulator @ ~80µA
Level shifter @ 1µA
MP3 decoder @ 5µA (in reset)
and the SD card
If you really want to safe as much power as you can, you will have to switch the power to the whole board off using a n channel fet with low rds-on and low-vgs.
And you will have to make shure that all data lines are low or tristate when the board power is switched off or you will have some parasitic feeding through the data lines.
Or you can use a buffer to tri state all lines together with the FET.
Edit:
There are SD card with much better idle currents, maybe you card has a power safe mode too.
Check if all data lines are low when the spark is in sleep mode.
Again, disconnect the data lines to make shure of that.
Maybe the sd-card goes into sleep mode if it does not see anny action on the spi interface for long enough.