My definition is far closer to a real smartphone than a simple OS and the fact that it plays music.
smartphone
noun
noun: smartphone; plural noun: smartphones; noun: smart-phone; plural noun: smart-phones
a mobile phone that is able to perform many of the functions of a computer, typically having a relatively large screen and an operating system capable of running general-purpose applications.
For your definition fine.
But don't try pretend there is only one definition or that it is clear cut in any which way or form.
Read up yourself - http://en.wikipedia.org/wiki/Smartphone