You could also look for 15VAC 30VA or something. (Toroids have less stray magnetic fields but higher leakage currents)
something like
this (windings in parallel... make sure they are connected in parallel correctly)
or
this (with windings in series) which is 18V 15VA
The best transformers are
R-cores but are a bit more expensive. These have very low mains leakage.
This particular one will provide you with 15V (use 2 windings in parallel) or 18V (2 other windings in series)
When loaded with the Atom there will probably be around 16VAC on it.
The object of the game is to have enough DC voltage to drop over the regulators.
The regulators output voltage is 15VDC so you need 18VDC
minimum on the regulator input and assuming ripple will be 1V or so, 19V DC or higher is perfect.
15V AC will give you about 20 VDC but in reality will probably be around 22 VDC in idle.
16V AC will give you about 21.5 VDC but in reality will probably be around 23 VDC in idle.
18V AC will give you about 24 VDC but in reality will probably be around 27 VDC in idle.
All of these options will work.
The higher the unregulated DC voltage is the warmer/hotter the regulators will become.
There won't be a difference in performance nor sound.