"Those of" is correct, but it sounds rather stilted to me. Also, are you sure that you are using "when" correctly in this sentence? The meaning it conveys is that you are reporting a time in the past, and at that past time, you hear yells and they remind you of WHEN your friend was drowning AT AN EVEN EARLIER TIME in the past.
This meaning is possible, but I suspect you may have intended to convey that the yells sounded like they came from your friend, and so she was drowning AT THE TIME you 'heard desperate yells'.
If so, the sentence should be:
"I heard desperate yells. They sounded like they came from my friend, drowning in the river."
It is grammatically possible to use "sounded like" to transfer the subject noun here, so you could also simplify the sentence to:
"I heard desperate yells. They sounded like my friend, drowning in the river."
Compare with: "Did you hear that? It sounded like Bob."
The latter is the most natural-sounding construction, in my opinion.
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