xsl beginner - sum () dont work

Where is the Problem.
Sorry im new to Xsl-Stylesheets

My Xsl-File:

<?xml version="1.0" encoding="iso-8859-1"?>
<?xml-stylesheet href="musik.xsl" type="text/xsl"?>
<!DOCTYPE musik [
  <!ELEMENT track (DiscID, Artist, Title, Album, Year, Comment, Track,
Genre, Filename, Filesize, Bitrate, Frequency, Layer, MpegVersion, time)>
  <!ELEMENT DiscID (#PCDATA)>
  <!ELEMENT Artist (#PCDATA)>
  <!ELEMENT Title (#PCDATA)>
  <!ELEMENT Album (#PCDATA)>
  <!ELEMENT Year (#PCDATA)>
  <!ELEMENT Comment (#PCDATA)>
  <!ELEMENT Track (#PCDATA)>
  <!ELEMENT Genre (#PCDATA)>
  <!ELEMENT Filename (#PCDATA)>
  <!ELEMENT Filesize (#PCDATA)>
  <!ELEMENT Bitrate (#PCDATA)>
  <!ELEMENT Frequency (#PCDATA)>
  <!ELEMENT Layer (#PCDATA)>
  <!ELEMENT MpegVersion (#PCDATA)>
  <!ELEMENT time (#PCDATA)>
]>
<musik>

        <track>
                <DiscID>1</DiscID>
                <Artist>...</Artist>
                <title>...</Title>
                <Album>...</Album>
                <Year>2003</Year>
                <comment></Comment>
                <Track>1</Track>
                <Genre>R&amp;B</Genre>
                <Filename>...</Filename>
                <Filepath>...\...\</Filepath>
                <Filesize>6.358.992</Filesize>
                <Bitrate>192</Bitrate>
                <Frequency>44100</Frequency>
                <Layer>3</Layer>
                <MpegVersion>1</MpegVersion>
                <time>104</time>
        </track>

        <track>
                <DiscID>2</DiscID>
                <Artist>...</Artist>
                <title>...</Title>
                <Album>...</Album>
                <year>..</Year>
                <comment>...</Comment>
                <Track>2</Track>
                <Genre>R&amp;B</Genre>
                <Filename>...</Filename>
                <Filepath>...</Filepath>
                <Filesize>2.496.597</Filesize>
                <Bitrate>192</Bitrate>
                <Frequency>44100</Frequency>
                <Layer>3</Layer>
                <MpegVersion>1</MpegVersion>
                <time>256</time>
        </track>
</musik>


My xsl-File:

<?xml version="1.0" encoding="iso-8859-1"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3c.org/1999/XSL/Transform";>
<xsl:value-of select="sum(/musik/track/time)" />
</xsl:stylesheet>