The startup chime is about 30k (looking at the copy Basillisk II extracted here). 8-bit, 22.5kHz, mono. You can also extract it from a ROM file using Audacity:

Set the project sample rate to 22050Hz

Import the ROM as raw data (unsigned 8-bit PCM, Big-Endian, Mono, 22050Hz)

You'll see the chime near the end of the file.

Crop away all the other data and save the file out.