This is perhaps not what you are looking for, but here is a link to a Python library I made that enables you to represent line graphs and scatter plots as sounds.

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I can add new features if needed, but how should statistical graphs be represented in sound?

Tactile graphics can help, but when presenting digitally, alt text is not enough when it comes to presenting data in an accessible format. You'll need to write a longdesc or provide a supplemental page of plain text/simple HTML that describes the graph and the data within it, exactly the same if you were telling someone about the data points verbally. Once you write down the way you would verbalize the data in the graphs and charts, you have exactly what we'd hear when encountering it with our screen readers. Break complex graphs and charts down, simplify where you can, and provide this extended description via a link adjacent to the graphic or directly within the page containing the chart itself.

I don’t know if there are any. Honestly tactiles are still the best way to approach this. You may have to change over to a more traditional approach for these students.