The Sandisk Sansa Clip music players used a SoC from AMS (Austria Micro Systems) which ran at around 40 or 48 Mhz if my memory is correct... installed Rockbox on it and played Opus and FLAC on it just fine. But, I think the SoC had some extra hardware decoding functionality.
As for is there a compression algorithm with less computational complexity ... I would investigate if the old AC3 and mpeg 1 layer 2 (or whatever was used in Video CD / SVCD and old SD satellite transmissions) are lighter, as the patents expired for them ... both would use more bitrate to get same quality as MP3 but should be a bit easier to decode.
I know of libMAD , a mpeg decoding library which does everything in fixed point so it may be suited for conversion to microcontrollers
This page has a bunch of projects and mentions using libMAD with Arduino but the examples in the page use helix :
https://www.pschatzmann.ch/home/2021/08/13/audio-decoders-for-microcontrollers/Found this appnote / document which shows using libMAD and LPC2148 but that's ARM7 chip and kind of expensive at around 15-20$ :
https://www.nxp.com/docs/en/application-note/AN10583.pdfMaybe could be used with a microcontroller that has less memory and storage and more reasonably priced.
but browsing the appnote they still had to tweak it a bit to fit in the 32 KB of ram the microcontroller has, and they only used mono mp3 files because there's only one built in DAC. Not sure if the chip had enough processing power to do two channels.
edit : Ah, on page 23 it says :
Libmad is a well-known library to decode MP3 files. Libmad is also used in the popular
free VLC player, however, as you can see in the last table, the porting of the actual
version of libmad
on an ARM7 (@60 MHz) loads the CPU from the 40 % to 70 % of its
power, so only one audio channel can be decoded in real time. Browsing the web we
found one interesting alternative represented by the Helix MP3 Decoder suitable to
decode, in real time, a stereo MP3 stream. Additional audio codec is also available, like
AAC. All these codecs have a specific RCSL and RPSL license. For more info about this
codec please refer to: