Alternatives to read mp3 without using an sdcard as source for the files?
The problem of all alternatives I can think of is a commercial one:Its more expensive than to waste space. So if you look up your facvorite online dealer you will quickly find out that e.g. buying the cheapest 1GB SD card costs double than buying the cheapest 32GB card (6$). So the more space you waste the cheaper. Get an used 512MB or 1GB card (cost in my country around 1$) and be happy with the space reserve.
Of course if saving space is a topic and not money get 20 x 1MB eeprom I2C boards (20*3$ from china), solder the hardware (you'll need some more parts to manage the board addresses), get a power source, write a library for managing the memory and after spending around 100$ and a lot of time you saved up to around 15.98 GB of free space but not the environment.