Audacity Spectogram view: how to see exact frequency and intensity under the cursor?
What you are asking is possible using heuristics but impossible in general because of the uncertainty principle. A single sample is just a value, it does not even make sense to ask about its frequency or frequencies. Roughly speaking (though not strictly true), the concept of "frequency" necessarily implies some form of repetition, thus it only applies to a range of samples, not a single sample. You can, of course, look at a fixed-length window around the sample you are interested in, but the result (both the number of frequencies and their accuracy) will depend a lot on the window length.

I think Sonic Visualiser has pretty decent heuristics for instantaneous frequencies.