Yes you can (this requires admin rights ! ).  

The symbols used are in conformity with international standards ISO 8253-1:2010.  If you don’t like these symbols used to display the thresholds in audiometry or the scores in speech audiometry, you can replace them with your own symbols.  For this, click on the format button in the ribbon, and then on the Show Legend button.  You can then click with the right mouse button on the symbol you want to change and replace it with the symbol of your choice.

Click on preview to see how this new symbol combines with the other symbols.

After saving, the new symbols will appear on the audiograms.