ToExpression@ImportString@ExportString[#, "XML"]&/@Cases[xml, XMLElement["math", _, _],[Infinity]]

<math xmlns="http://www.w3.org/1998/Math/MathML">
        <apply>
          <ci> MM_e</ci>
          <ci> mem </ci>
          <ci> k </ci>
          <ci> sp_3 </ci>
          <ci> sp_1 </ci>
          <ci> Km </ci>
          </apply>
      </math>

ToExpression::esntx: Could not parse
FormBox[TagBox[RowBox[{MM_e,RowBox[{(,RowBox[{<<9>>}],)}]}],MathMLContentTag,AutoDelete->True],TraditionalForm] as input.

MM_e(mem,k,sp_3,sp_1,Km)

MM_e(mem,k,3 sp_,1 sp_,Km)

DisplayForm@ImportString@ExportString[#, "XML"] & /@Cases[xml,XMLElement["math", _, _], [Infinity]]