<MPD mediaPresentationDuration="PT7M57.72S">
    <Period>
        <AdaptationSet mimeType="audio/mp4" segmentAlignment="true" startWithSAP="1">
            ...
        </AdaptationSet>

        <AdaptationSet mimeType="video/mp4" segmentAlignment="true" startWithSAP="1">
            ...
        </AdaptationSet>
    </Period>
</MPD>

using ptree_t           = boost::property_tree;
using shared_ptree_t    = boost::shared_ptr<ptree_t>;

shared_ptree_t GetAudioAdaptSet(shared_ptree_t & spMPDtree)
{
    using namespace std;

    shared_ptree_t spAudioAdaptSet;

    auto range = spMPDtree->get_child("MPD.Period").equal_range("AdaptationSet");

    for (auto itr = range.first; itr != range.second; ++ itr)
    {
        if (boost::iequals(itr->first, "AdaptationSet") &&
            boost::iequals(itr->second.get<string>("<xmlattr>.mimeType", ""), "audio/mp4"))
        {
            spAudioAdaptSet.reset(new ptree_t(*itr));
            break;
        }

    }

    return spAudioAdaptSet;
}