The procedure of memory upgrading to the EII+ status is quite complicated, and is described in the "EmulatorII1MBMemoryUpgrade".pdf. You can download it on the Emulator II List Yahoo group (files, technical). As for the crackling in sound, perhaps you could try to run the Emulator II ZM325 Memory Test Disk... The image of the test disk could be downloaded here:
http://www.emxp.net/ (EII additional downloads...)
How to interpret the results of the memory test is described in EII Memory Test.pdf (Emulator II List Yahoo group (files, technical).