Upgrade ZX Spectrum 48k with 128kB of RAM and play all the games you couldn't play before. The sound will stay as before, but you can always make an AY interface to sound like ZX Spectrum 128k.
In order to make the RAM expansion work, a small modification to your ZX Spectrum 48k is required to disable the 32k upper RAM. For Issue 2/3/4 connect pin 5 on IC23 (74LS32) to pin 14 (+5V). For Issue 5/6 connect pin 35 on IC27 (ZX8401) to pin 40 (+5V). Both mods will hold the CAS/RAS signal high so it won't reach the RAM chips.
https://spectrumcomputing.co.uk/forums/viewtopic.php?t=2616
With this mod without the RAM expansion, your ZX Spectrum 48k will work as ZX Spectrum 16k.
To test the RAM expansion use modified zx-diagnostics that allows you to manually select 128k test regardless of the ROM.
Original page: https://velesoft.speccy.cz/zx/external_128kb_upgrade/index.htm