Here's how I do it, along with a check for giving an alternate message if there is nothing to display.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xsl:Stylesheet [ <!ENTITY nbsp "&#x00A0;"> ]>
<xsl:stylesheet

    version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:msxml="urn:schemas-microsoft-com:xslt"
    xmlns:umbraco.library="urn:umbraco.library"

    xmlns:Exslt.ExsltMath="urn:Exslt.ExsltMath"
    exclude-result-prefixes="msxml umbraco.library Exslt.ExsltMath">


<xsl:output method="xml" omit-xml-declaration="yes"/>


<xsl:param name="currentPage"/>

<!-- ============================================================= -->

<xsl:template match="/">
    <!-- SELECT THE DOCUMENT TYPE ALIAS TO SEARCH FOR -->
    <xsl:variable name="documentTypeAlias" select="'multimedia_file'"/>

    <!-- GET ALL NODES OF THE SELECTED DOCUMENT TYPE ALIAS -->
    <xsl:variable name="allIds">
        <xsl:for-each select="$currentPage/ancestor-or-self::node[@level=1]//node[@nodeTypeAlias=$documentTypeAlias]">
            <xsl:copy-of select="." />
        </xsl:for-each>
    </xsl:variable>

    <!-- COUNT THE NUMBER OF NODES -->
    <xsl:variable name="numItems" select="count(msxml:node-set($allIds)/node)" />

    <xsl:choose>
        <xsl:when test="$numItems > 0">
            <!-- SELECT A RANDOM NODE -->
            <xsl:variable name="randomNumber" select="floor(Exslt.ExsltMath:random() * $numItems) + 1"/>
            <xsl:variable name="randomItem" select="msxml:node-set($allIds)/node [position() = $randomNumber]" />

            <!-- NOW THAT YOU HAVE A RANDOM ITEM, YOU CAN DISPLAY IT -->
            <xsl:value-of select="$randomItem/@nodeName" />
        </xsl:when>
        <xsl:otherwise>
            <p>No item found</p>
        </xsl:otherwise>
    </xsl:choose>
</xsl:template>

<!-- ============================================================= -->

</xsl:stylesheet>

cheers,
doug.