I have an ASUS M3A78PRO motherboard with realtek 7.1 audio chipset onboard. I have also two speaker and a subwoofer with no external volume control. I'm on Windows 7. There is a way to control, separately from speaker volume, subwoofer volume?
This isn't really an easy thing to answer. We would need to know some details about your "sub-woofer" to be clear.
Most 2.1 speaker sets (left/right + sub) typically do not have a separate
You can clearly see that there's only 2 channels. In this case, the only way to amplify the sub-woofer's volume, is to use an equalizer and amplify the range that the electronics filter out for the subwoofer. Cheaper speaker systems frequently continue to pass that range onto the left & right speakers, so you'll also be amplifying them.
Some low-grade speaker systems have additional plugs for TRUE 3.1/5.1/7.1 configuration options, where you end up with 2/3 or 4 of those 1/4" jacks. For example, in basic 3.1 surround sound, you would end up with 1 plug carrying the left and right channels, 1 plug carrying the center and subwoofer channels. In those cases, you simply turn up the volume on the specified channel.
Higher-end speaker systems, rather than relying on 2,3 or 4 separate plugs will usually offer either an optical interface or digital interface where the channels are carried as a single digital signal and then the receiver interprets the signal and then sends the signal to each speaker. Their plugs look like this:
Your system, does indeed include a digital S/PDIF port for hooking up to a digital receiver... but I doubt your speakers would support it. (unless you paid quite a lot for 'em) The 6 "1/4 inch" jacks on your motherboard can be configured to have 4 of them behaving as audio out, giving you 7.1 audio. (left/right front, left/right mid, left/right back, center and sub) Usually there's a tool that pops up when you plug something in that asks you what you just plugged in.