Songs will be rated 1-5, with 1 being awful, 3 being mediocre, and 5 being excellent.

Marathon 1 was the only game in the Marathon series to back background music (save for the title music in the others).  I loved this and was minorly disappointed at Marathon 2 and Marathon Infinity for the lack of it.  Here's my review of a particularly loved, npstalgix game's soundtrack.

Aliens Again - At first it doesn't seem too special - just some whining voice-like instruments, but then the drums and supporting string kick in... I used to sort of dislike this song because it was repetitive, but I've actually come to enjoy it, especially the intro.  Also, I have good memories of the majorly creepy level that greeted me with this music.
> 4 (Good)

Chomber - Something weird about the Marathon 1 songs is that, for the most part, they're very repetitive.  This song's claim to fame is its organ riff that repeats, which I like a lot.  It's also joined by the voice-like instrument that the author seemed to like.  Fairly decent.
> 4 (Good)

Fat Man - I love this song!  It starts out with this really catchy, fast organ riff that repeats and is then replaced by a guitar-like instrument playing the same tune.  The part where the drums kick in is memorable, but what really sets this song apart from the others is the organ solo that comes in at about the 2/3 mark.  It plays up and down, and is very jazzy.  I'd like to someday learn how to play that.
> 5 (Excellent)

Flippant - This has long been one of my favorite songs.  With its fast hi-hat and low piano, it sticks in your head.  Later strings replace the piano, and that voice instrument comes in with a melody.  My love for this song is augmented by the fact that the only level in Marathon 1 where it's played happens to be my favorite level in the game.
> 5 (Excellent)

Flowers in Heaven - Ah, the first atmospheric song.  I really like this one.  It's very soothing with its soft string and wind instruments, but it also has this kind of eerie feel to it.
> 5 (Excellent)

Freedom - I always giggle like a girl when I look at the name of this song, because the "freedom" is referring to Durandal (the rampant AI), who is a very amusing character (albeit the antagonist for a while).  Still, it's one of the more repetitive songs.  The main tune plays in two different keyts, alternating between two similar, low instruments, with kick and hi-hat drums in the background.  As a song, it's all right.
> 3 (Mediocre)

Freedom - Another one of my favorites!  This one always seemed kind of angelic to me, starting out with a high repeating string line, then a low one that plays long notes up and down an octave, finally accompanied by distant melodic voice-like chords that fade in and out.  It is one of the more majestic songs.
> 5 (Excellent)

Landing - This is definitely one of the creepier songs.  I think it played in the first level of the game (where it's dark and you're not sure what's going on).  It starts out with this really low version of that voice instrument, playing three long, repeating notes, then joined by floating woodwind notes that are sometimes dissonant, which provides a very uneasy feel to the song.  Quite atmospheric.
> 4 (Good)

Leela - This song starts out with a low, repeating string, that occasionally rises in pitch, which is joined by a high, floating wind instrument similar to that of "Landing," and later by sporadic kick drums and tamborine shakes.
> 4 (Good)

New Pacific - This has been a new favorite of mine; it's an eerie, atmospheric harmony of low strings, fading woodwind sounds, and the voice-like instrument previously mentioned, which I think works really well in this song.  A good listen on a beach at night!
> 5 (Excellent)

New Pacific (Reprise) - This "reprise" of the previous song is kind of like a softer version of the song, with soothing (less eerie) ambient instruments.  A little way into the song there's some quiet horns in the background, which give the song a unique song.  It's quite a pleasant listen.
> 5 (Excellent)

Rapture - This is the epitome of Marathon 1's characteristic voice-like instrument.  While it plays high, choir-like sounds, woodwind and low strings alternate in the background.  This is another one of those songs that sounds rather angelic.
> 5 (Excellent)

Rushing - Probably the most drum-rhythm oriented song of the group, this song starts out with an echoing, descending string loop that is quickly joined by kick drums and cymbols, followed by a choir-like accompanyment, with little bells in the background.  It's very much an action song, and very enjoyable.
> 5 (Excellent)

Splash(Marathon) - Starting out with a low, rapid voice-like loop, this song quickly kicks it into high hear when it introduces the kick drums and hi-hats, as well as a string, which plays a high melody that has a lot of variation, and occasionally goes on a little rampant crescendo.  There are also parts where a high, angelic wind instrument of some sort is played.  What sounds like an oboe also joins during the last third of the song.  It's a very intense, interesting, and invigorating song.
> 5 (Excellent)

Swirls - I think this is song is very strong feel of melancholy to it.  It also has a strange sedative effect; whenever I listen to it I feel like I'm going to fall into a deep sleep... all the instruments are kind of muted and soft; you really have to listen to experience.  It's very repetitive and lullabic.
> 4 (Good)

What About Bob? - The first 30 or so seconds of this song is a very slow piano metronome, which then is joined by building strings that play a continual melody.  There's... not much to say about this one.
> 3 (Mediocre)

> Average Score: 4.4

Some of the songs in Marathon 1 were very well-made, and others were just okay.  Either way, they made for an enjoyable background to the game, and are fun to listen to for the most part.