You don't actually say how you want it sorted. Try
<xsl:template match="combo">
<xsl:for-each select="option">
<xsl:sort select="@value" datatype="number"/>
<xsl:copy-of select="."/>
</xsl:for-each>
</xsl:template>
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference