STOP BEEP SOUND WHILE SUBMITTING FORM

I have many controls on my form, when i use enter key to submit the form i get beep sound however submit works fine, please let me know how to stop that beep sound also i am not using type="submit", instead onclick is used when user clicks form gets submitted. While click no problem everything is fine but on ENTER key press beep sound comes.
Below is code i am using

function catchEnter(e)
{
if (!e) var e = window.event;
if (e.keyCode) code = e.keyCode;
else if (e.which) code = e.which;

if (code==13)
{
//alert(document.filterForm.from_date.value);
//return validateDate(document.filterForm.from_date);
if(validateDate(document.filterForm.from_date))
{
return SubmitPage('NU');
}
else
{
alert("false");
}
//alert(return validateDate(document.filterForm.from_date));
//document.forms[0].onsubmit = returnFalse;
//QuickSearch();
return true;
}
return ;
}