[Huwstan (OP)]

why do certain notes in music, when played all at once sound nice for example a C major chord of C,E,G will sound nice but if you play some notes together they become dissonant? same works for melodies, if a note is played out of key, it sounds horrid, but that note will sound good if played in a different scale/mode etc so its not that some frequencies of sound are nice to our ears, just certain intervals??

[Soul Surfer]

When you hear a musical note played it is not just one frequency but contains a range of harmonics and subharmonics that are multiples of the main frequency of the musical note.  When you hear two or three musical notes played together you not only hear all the harmonics and subharmonics but also hear all the frequencies made by adding together and subtracting all the frequencies from each other.  In the notes that sound nice together all these frequencies add up and subtract to form frequencies that fit into the harmonic and subharmonic pattern.  Then as you play notes that sound less nice together you get more and more interfering frequencies in these additions and subtractions and the patterns become more and more complex eventually degenerating onto noise

[Dewitback]

So are you saying that one note holds alot of different notes that we cannot hear seperatley?

[Soul Surfer]

A pure sine wavethat lasts an indefinite length of time has only a single frequency.

Most musical notes are not pure sine waves and start and stop this introduces a range of harmonic frequencies in them.  Fourier analysis of the sound wave will reveal them.

We can hear these harmonics seperately because they are what gives the musical note its quality  ie a flute, an oboe, a violin and a piano playing the same note all clearly sound different even thoufg the fundamental frequencies are the same,

[eric l]

So are you saying that one note holds alot of different notes that we cannot hear seperatley?

One note from a musical instrument or human voice holds a lot of frequencies.  They are multiples of the base frequency and known as harmonics.  In fact, it is by these harmonics that we recognize one voice from an other, or a saxophone from a trumpet...  Single frequency sounds do exist, a tuning fork without resonance box comes very close.