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In case anyone ends up here, like me, experiencing this problem on a more recent Mac OS (in my case Sierra): None of the other solutions here worked for me. Sierra doesn't give the option to connect in headset or headphones mode, and the bitpool setting made no difference.

I finally figured it out: Going to sound preferences and manually changing the sound input device back to Internal Microphone seemed to switch my headset into headphone mode, making the sound quality worlds better.

Note that you can also do this faster by option-clicking on the volume icon in the menu bar.

Hope that helps someone out there.

Make sure that when you pair the headset you select "use as wireless headphones" (or similar) instead of "use as headset". The headset profile was designed for phone calls, and you should only use it if you're pairing a headset that does not support A2DP.

Your Plantronics headset probably supports both (if it has a microphone) because some devices switch between the two profiles for music-listening and call-taking.

I have a Motorola A2DP headset and on occasion with it pairs in headset mode with my MacBookPro it's very staticy and the sound is sub-par. Usually, by disabling bluetooth and then turning it back on and resyncing everything it works fine again.

It's pretty rare that it ends up with bad sound, but I get the feeling that Apple has pretty crappy bluetooth drivers.

Try the method explained by Casey Liss
in this post at Casey's Tumblelog. 
It tweaks the bitpool setting which makes all the difference in the world.

UPDATE:

The above blog post points to an article on Scott Dier's blog
that no longer exists. 
However, it can be found on The Wayback Machine here, and it says:

OSX, Snow Leopard, a2dp

So you'd figure with Snow Leopard that sound quality with some a2dp headsets
would be better — turns out it isn't. 
I was able to 'fix' my sound quality with a Motorola S9
by going to Bluetooth Explorer (option-click on bluetooth to find it)
and going into Utilities->Special Options. 
I changed the bitpool minimum to 40 and the sound quality is MUCH better. 
Sounds like its a problem with negotiation between this headset and OS X. 
If you set the value too high
OS X will let you know that the headset rejected the codec settings. 
I'd figure that 40 is ok for me,
but might not be ok for others — use this at your own risk.

Posted by Scott Dier at 11:39 AM (Wednesday, October 7, 2009)

Ok You are on Leopard which supports A2DP, as opposed to non-stereo on Tiger so that should not be the problem.

Some headphones have a headset mode and get connected in that mode incorrectly and sound awful, so you should endure that your headphones are not being connected in headset mode.

You should also try your headphones with another computer to ensure that you bluetooth adapter is not faulty.

While I tried a lot of these options that everyone is presenting as solutions, I ended up fixing my headphones a different way. I went to system preferences on Mac and then went to Bluetooth. From there I right clicked on the device (Beats Solo3 Wireless) that was having issues and clicked "remove". After I did that, I reconnected them and now they work fine. My initial problem with the headphones wasn't on an individual platform rather a problem on all platforms within my mac, which includes Spotify, Youtube, FaceTime and anything else you can think of. This will give you another option to try and I hope it helps.

I was having the same issue with a pair of Coby E7 bluetooth headphones where they sounded muffled. I went to system preferences and realized that while the audio was playing through the headphones they weren't listed under Audio -> Output as the output source.

I decided to try and disconnect the headphones by going to System Preferences -> Bluetooth and clicked the "X" button to remove them. I rebooted the headphones and repaired them and the issue is resolved and now the headphones are listed as the output source under sound.

Hope this helps.

If you have SoundFlower installed, try uninstalling it. 
That solved this for me.

Use the SoundFlower uninstaller script located in the original SoundFlower install .dmg file.

The top answer by linesarefuzzy worked partly for me. 
In addition to switching to internal microphone,
I also had to disable dictating (otherwise it had no effect).

To disable dictating, go to: System preferences -> Keyboard -> Dictating, and switch dictating to off.