I am so sorry I was not able to answer this post earlier re heart murmurs...
Unfortunately the problem is not uncommon in the breed. Thankfully many Chihuahuas live long lives with he condition, there is some really good medication available now.
Obviously your vet doesn't think it necessary to begin treatment now, but do keep a close watch on your little one.
I will briefly share with you my Honey's experience of heart disease, not to worry you, but just so that you are aware of what may
happen in some dogs...by no means all!
Honey was diagnosed with a grade 3 murmur and within a couple of weeks began to breathe faster than normal. She began medication, but sadly deteriorated and died within 6 weeks.
Signs to watch out for are laboured breathing, panting, fainting, loss of appetite and in Honey's case she lost tufts of hair...vet said not related, but I believe it was and linked to poor circulation :
I really hope your little one lives a long and happy life and there is no reason why not for most dogs...my little Hun was almost 10 when she passed.
Your dog possibly has mitral valve disease, but ask vet for proper diagnosis. If you have any questions, the vet nurse at surgery should be able to help you.
If I can find the links I had when Honey was diagnosed will post them later...or if I find anything that might help. A really healthy diet will go a long way to help, moderate exercise, but again, check this out with vet
Hope all goes well and love to you and little one.