Does ATX motherboard has anything to do with cooling

The atx standard is only longer downwards vs microatx to have more expansion slots. (9.6" x 12" vs 9.6" x 9.6") So the cpu and ram area is about the same. The extra slots may help cooling when you have multiple graphics cards. Sli/cf on matx tend to have the cards right on top of each so the top card doesn't get much air. While atx will usually have another slot in between letting it have more air and thus be cooler.

Here are just some random examples off of google.

with a single graphics card and a sound card do i need atx motherboard

Nope, given you buy a PCI-E sound card, I'm sure it'll fit into an x4 slot, which Micro-ATX most definitely has.

can u say how much power phoases these motherboard has
ASUS P8B75-M
asus P8B75-M LE

More phases does not mean better, some mobo phases are better than others. It's the same company so would probably be the same phase type, but you aren't ocing so it doesn't matter.

Those mobos have 2 slots in between the pcie slots so having a sound card would be fine.

does lesser power phases affect cooling for ex 3+2 vrm's

No cooling would be the same. You are not ocing, phases really don't matter.

i have chosen three of b75 motherboards which one is best.
Gigabyte GA-B75M-D3H
ASUS P8B75-M LX
MSI B75MA-E33(pls tell me whether this board has lucid virtu)
i want better cooling,best reliability,best performance
specs are
intel core i5 3570 3.4ghz
corsair vengeance 2x4gb 1600mhz
asus gtx 550ti 1gb
asus xonar ds sound card
i will be adding a 128gb ssd in future for the boot drive
here are some info's i got
majority 3+1 = supports 95 watts
some 4+1 = supports 125 watts
some 4+1 = supports 140 watts
my i5 processor is 77w TDP but i think on max stress the TDP increases.so what about ram i think it also requires power phase

Case and case fans and cpu cooler will have a bigger effect on cooling than any mobo.

The tdp will not be higher because you are not ocing. The +1 is the ram power phase. Now stop thinking about power phases. The gigabyte or asus would be my choice. The GA does have 4 ram slots though.

so can i go with gigabyte board.does this board has lucid virtu so that i can switch over discrete gpu and onborad gpu

None of them have virtu. Why would you want to switch between gpus?

Yeah, if you have a discrete card, why would you bother with onboard?

for faster video conversion.