Using the default backup recovery function of Quickbooks, it should be possible to recover QBMB files and then export the data in a QBM file. Other than that, there is probably no other way how to achieve this.
Convert QuickBooks backup file to QuickBooks portable company file.
We found 2 software records in the database related to .qbmb to .qbm file format conversion.
Microsoft Windows software - convert qbmb to qbm on Windows
A business accounting software from Intuit
Convert from qbmb file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned qbmb to qbm. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert qbmb to qbm on OS X
An OS X version of popular accounting software