Try a different headset -- some headsets have higher impedance than others, making the sound lower than you'd like.
I do not see in your post everything that you have tried so far, so I'll just start with the beginning: please keep in mind that your Optimus has several different volume settings. The volume for your wired headset is a different setting than the volume for your bluetooth headset, for example. Media, ringtone, notifications, calls, and system sounds can all be on different levels. Please make sure the volume is up for the application you are using.
I use the Samsung headset that came with my Samsung Moment. It's getting pretty good results with acceptable sound quality. With this headset, having the volume set to the maximum is uncomfortably loud. The Samsung headset that came with the Samsung Instinct HD sounds like it has a little more static, but is still good on volume.
I have Sony headphones (MDR-7506) which have the best sound quality of the three, but is a little quieter. With this headset, I can set the volume to the highest level, if I wanted to drown out all outside sounds.
On a similar matter, I noticed that the MP3 files that I ripped from my CD's are a lot quieter than listening to downloaded MP3 files or Pandora, for example.
In conclusion, the low volume may be a combination of the volume setting on the phone, the headphones being used, and the media being played. If you don't want to troubleshoot which aspect is causing the low volume, there is an app called "volume boost" on the Android market, which may help provide a solution. I tried the trial version, and it sounds like a noticable boost in volume.
Ive tried changing headsets and tried the app and it still sounds bad.
Is the volume too low, or does the phone sound bad? In the original post, there was an issue with the volume. In the most recent post, it sounds like you are concerned about quality. I admit that the sound quality isn't as good as some of the other phones I've owned in the past. By hooking it up to a radio, or using quality headsets, I notice a significant improvement in sound quality, but volume is sacrificed. However, since the volume is satisfactory, it's not such a bad sacrifice.