What kind of circuit might be required to to play a particular piece of music without an SD card.
For example, a toy car playing a Christmas song the moment it is switched-on. It doesn't require an SD card.
How complex will it be?
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\$\begingroup\$ There are sound modules with a lot of common tunes out in the market. They are extremely small, and cheap — typcially $1 for single pieces. You only have to hook up a battery and a speaker. \$\endgroup\$– JankaFeb 26, 2020 at 1:01
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\$\begingroup\$ You can use any memory device to store music. SD cards are convenient for one-off projects for many reasons. They make millions of cards, so they are probably using mask ROM, which will cost a lot of money once to set up, but then it's cheaper to make each card. \$\endgroup\$– user253751Feb 26, 2020 at 12:46
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3 Answers
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How much definition or detail will determine how much and what type of memory is needed. Music could be played with methods varying from simple MIDI tunes requiring very little memory to varying compressions of soundwave files. In some cases the same type of memory as you find in an SD card might be present, but not necessarily in SD card form. Many types of memory exist that are suitable for low end audio. For a simple "beep boop" type of song to play as little as a few dozen bytes might be used and varying types of rom would be possibilities.
As for the type of circuit that can do such a thing, you need a device to produce vibrations, usually a speaker. You need a device that can drive current through the speaker at different frequencies(essentially a processor), and you need something to store the list of frequencies to play at what time(essentially memory). Many variations of each of these devices exist.
The tune may be recorded as simply as a list of notes and the frequencies to be played on that note and the duration. Some of this may be done in hardware for a very simple device. A wave file(uncompressed audio) is simply a list of amplitudes of the sound wave at increments of time.
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You could try using an um66 or bt66 ic. It looks exactly like a transistor and plays a song when you power it and connect it to the speaker. You dont need an additional amplifier to increase the loudness. These guys cost less than a dollar. 
answered Feb 26, 2020 at 6:40
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There are greeting cards that support standard songs (like Christmas carols), and some that even do custom songs. One could excise the guts from such a card and transplant it into a toy.
Or you can buy these things as standalone module in the low single-digit dollars range.
answered Feb 26, 2020 at 0:54