Here is a list of songs that I feel describe the relationship between Mulder and Scully or characterize one or the other's personality. A few of the songs have been played on the series and you'll recognize them. At the end of the lyrics I have a brief summary of why I feel this song can be categorized as having significance.

500 Miles
After All
Amazing Grace
Annie's Song
At The Beginning
Because You Loved Me
Bizarre Love Triangle
Book Of Days
Building A Mystery
Candle On The Water
Can You Feel The Love Tonight
Can't Help Falling In Love
Come To My Window
Crash and Burn
Died In Your Arms
Dust In The Wind
Eternal Flame
Every Little Thing She Does Is Magic
Everything I Do I Do It For You
Fallen Angel
Foolish Games
For Your Babies
Full Of Grace
Half The World
Head Over Feet
Heaven Is A Place On Earth
Hey You, It's Me
How Do I Live?
If I Never Knew You
If We Hold On Together
I Am A Rock
I'm A Believer
I'll Be
I'll Come To You
I'll Remember
I'll Stand By You
It's All Coming Back To Me Now
It's Only Make Believe
I Will Carry You
I Would Do Anything For Love
Joy To The World
Lean On Me
Lethal Weapon
Making Love Out Of Nothing At All
No Place That Far
Now And Forever
One More Try
The Promise
Promises In The Dark
Right Here Waiting
The Rose
Save The Best For Last
Soul and Inspiration
Stand By Me
Standing Outside The Fire
Storybook Love
Take My Breath Away
There You Are
Time After Time
Total Eclipse Of The Heart
True Colors
Truly, Madly, Deeply
Twilight Time
Walk With Me
Walkin' In Memphis
We Belong
Wind Beneath My Wings
When You Say Nothing At All
When You Walked Into My Life
You Are Not Alone
You're Still The One
You've Got A Way