SOME EXAMPLES OF AY chip MUSIC FOR ZX SPECTRUM 48K
Firts AY interfaces for ZX Spectrum 48K appeared actually even before 48K model apeared because they were originally for ZX81 computers but it was also possible to connect them into ZX48K. But don't buy the early devices (like Fuller Box) as they are not standard nowadays. Modern AY interfaces (fully compatible with Spectrum 128K) appeared later, mostly in Central Europe, and their base was Czech'o'Slovakian MELODIK interface.
Well, actually Spectrum 48K can play all chiptunes written on Spectrum 128K and some digital songs (Amiga MOD-like music) that are big up to about 38Kb and don't frolic on RAM areas higher than 48Kb. However, ZX48K - in contrast to 128K - is STEREO in most cases because AY interfaces became popular several years after releasing 128K series.
Here is a list of GAMES WORKING ON ZX48+AY!!!
AY music editors/processors made especially for ZX Spectrum 48K are:
Sound-Tracker 1.1 (for chiptunes) SQ-Tracker (also for chiptunes) Sample-Tracker 2.0 (for digital music)
There are many sites about AY music so let's mention just several musicians who still contemporary make their songs
on ZX48K:
Some classic music from ZX Spectrum 48K with AY interface:
Hacker Kicia - SLL3 ZX Cover (1990)
Hacker Kicia - Amiga cover (1990)
Ziutek - Kaz 6th demo song (1990)
Digital AY music from ZX Spectrum 48K You can find below.

AY-interface for ZX Spectrum 48K MELODIK

MELODIK MELODIK inside

ZX Spectrum 48K, AY-interface, joystick interface and divIDE flash-cards/HDD reader
Brand new (2009) AY and YM interfaces for ZX Spectrum 16K / ZX Spectrum 48K / ZX Spectrum 48K Plus / Timex Computer 2048 made by ZAXON
(click to enlarge)
You can buy them HERE!!!!!
SOME EXAMPLES OF DIGITAL AY MUSIC FROM ZX SPECTRUM 48K:
YERZMYEY/AY-RIDERS - "Dig It!"
Music from Interama's "MAGIC SHOW" demo made in 1996 for ZX Spectrum 48K
...like above.
Flash Tracker Demo by Flash Inc,