The complete XML skeleton for getMultipleLeads is (you may need to re-ident it to improved readability)
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:mkt="http://www.marketo.com/mktows/">
<soapenv:Header>
<mkt:AuthenticationHeader>
<mktowsUserId>?</mktowsUserId>
<requestSignature>?</requestSignature>
<requestTimestamp>?</requestTimestamp>
<!--Optional:-->
<audit>?</audit>
<!--Optional:-->
<mode>?</mode>
</mkt:AuthenticationHeader>
</soapenv:Header>
<soapenv:Body>
<mkt:paramsGetMultipleLeads>
<!--Optional:-->
<leadSelector/>
<!--Optional:-->
<lastUpdatedAt>?</lastUpdatedAt>
<!--Optional:-->
<streamPosition>?</streamPosition>
<!--Optional:-->
<batchSize>?</batchSize>
<!--Optional:-->
<includeAttributes>
<!--Zero or more repetitions:-->
<stringItem>?</stringItem>
</includeAttributes>
</mkt:paramsGetMultipleLeads>
</soapenv:Body>
</soapenv:Envelope>