.toString(radix) ?

I know that you do not have responses ending with /n. But there is or are some thing(s) equivalent from which you can derive when a response is completely received.

I took a look at the wiki of the device: https://www.dfrobot.com/wiki/index.php/V­oice_Module_SKU:_DFR0534

The response is always: <0xAA><cmd><noOfResponseBytes><rspByte0>­<rspByte1>...

Together with the pre-amble of 0xAA we can detect begins of a new command response in any string (We can safely assume that no response value has AA w/ a valid command as next byte in the response value(s), and to start with, we omit checking for valid command valid response data lengths against lookup tables).

I can see something like this:

// --- process response
function proc(rsp) {
  // response processing code goes here...
}

// [0] [1] [2] [3]
// AA  01  01  <statusValueByte> - status response
var cAA = String.fromCharCode(0xAA); // hex AA char
  , pAA = 0   // pos of AA +1 (+1 to make check easy)
  , dta = "", // received but not yet processed data
  , dL        // dta length
  , rDL       // response's data length
  , rsp       // response
  ;
s.setup(9600,{rx:B6, tx:B5});
s.on('data', function(d) {
  dta += d;
  if (    (pAA=dta.indexOf(cAA)+1)  // --> cmd byte if contains 0xAA
       && ((dL=dta.length)>(++pAA)) // --> response length byte
       && (dL>=pAA+1+(rDL=dta.charCodeAt(pAA))) // contains resp
     ) { 
    rsp = dta.substr(pAA-2,3+rDL); // complete response
    dta = dta.substr(pAA+1+rDL);
    proc(rsp);                     // process response
  }
});