Onboard Speakers Only Play One At A Time

I had an issue recently where I upgraded from OEM Vista x86 to Windows 7 Professional x86. There was very little sound coming from the internal speakers. I also had major BSODs from then, so obviously I was focusing my attention on the BSODs as a priority. That issue has been resolved, however, or, at least, I implemented a workaround - reinstalling Vista. I no longer have BSODs. The audio, however, is still extremely quiet. I've checked the following:

- Went into properties and maximised the 'levels' on everything.
- Went into volume mixer and maxed everything.
- Went into Devmgmt.msc and tried updating drivers - drivers already up-to-date.

I have two internal speakers as well as what I guess is a subwoofer (I'm terrible with hardware - its icon looks like a box server or something. I'm currently at work so I'll upload screenshots when I get home). Only one speaker plays at a time. I can change between the speakers by right clicking and saying "set as default speaker" and that plays sound out of one at a time with no problems, unless I choose the subwoofer(?). Is there a way to make both speakers play at the same time, instead of just using one of them?