WMP 10 like previous versions only has the volume controller at the bottom of the window when it is running. Windows itself has a mixer of it's own you can adjust by going to the accessories\entertainment section off of the Start\Programs menu and clicking on the advanced tab. If the sound card in your signature has a mixer in the software you can easily adjust levels there.
Is this from playing several songs in a playlist? If so, it could just be the way each individual song file was coded and it's bitrate. If it has a low bitrate, the quality of it will sound worse than something with a higher bitrate.
Forget quality and what not, some songs were just recorded too quiet, others too loud I bet about anything if you get Audacity, you could go and amplify the music you think sounds more quiet then re-encode.
Any prerecorded files you play will definitely see various levels just as much as if you had to turn up a receiver or portable radio when the next song is the ssslowww oooneee where the levels are far softer. Recordings you make on the other hand can be tamed with an input mixer to trim the highs or lows there. Automatic level controls can be a plus.
WMP 10 and other players just don't seem to have that unfortunately.