I didnt really see much of a way around not xraying her to be honest, however at the time you took her to the vets in the first place its fairly safe to say she was probably in pain with the problem so her heart rate may have been increased due to that thus exaggerating her murmur sounds. Or now that she is on heart meds (what type by the way ? ) they may actually be helping her heart.
Satisfy yourself that they will be using the safest type of anaesthetic possible for her condition and age, personally i'd want them to use rapinovet (propofol generic name) and not a combination sedative mixture which although can be partially reversed causes the heart rate etc to be depressed.
I presume if she is booked in they will have told you to starve her, many still give the ancient advise of also taking away water the night before so if that has been the case please do not do so
water only needs to be taken away in the morning
Remember -- your cat, your money & your worries so ask ask ask questions before hand and make sure you have had everything explained and are happy with the proposed procedure