As of this commit, I have now added cassette support for the AstroBASIC cartridge: https://github.com/mamedev/mame/commit/e23c64eb76c1ab9a1465d0101272c9df058bddad

Here's a screenshot of Bob Ogdon's "Anti-Aircraft Gun" tape in the process of loading:
[Linked Image]

And here it is running:
[Linked Image]

Here is George Moses's "Auld Lang Syne" song from the Christmas tape after having loaded:
[Linked Image]

I have also added tape output support. Here is a screenshot of a small BASIC program I wrote (as you can tell, I am woefully unfamiliar with the Astrocade's control mapping), having been loaded off the tape to which I saved it in a previous run of MAME:
[Linked Image]

And here it is running, to verify that the data was loaded correctly:
[Linked Image]

For further proof, here is the tape image that MAME generated:
http://polygonal-moogle.com/mame/astrocade/test.wav

Please note, support for the cassette interface is available through MAME's "Software List" functionality only. That is, the AstroBASIC cartridge ROM will need to be in a ZIP file called "astrobas.zip", with a path of "roms/astrocde/astrobas.zip", or wherever appropriate for where you have configured MAME to expect software-list ROMs to be found. Rather than loading AstroBASIC as a loose file through the -cart switch, one specifies a software list entry like so:

Code
mame64 astrohome astrobas <options>


Thank you for the informative bug report on MAMETesters, and thank you also for posting your request on these forums, as I am not in the habit of checking MAMETesters.