Can an audio circuit be powered by a switched-mode power supply?
Let me give you a little background on myself... I've been working professionally in the audio industry for more than 14 years. I've designed circuits for most of the major pro-audio companies, one audiophile company, and several consumer audio companies. The point is, I've been around and know a lot about how audio is done!
SMPS can and are used for audio circuits! I've used them from sensitive microphone preamps to huge power amplifiers. In fact, for the larger power amplifiers they are mandatory. Once an amplifier gets over a couple of hundred watts then the power supply needs to be super efficient. Imagine the heat produced by a 1000 watt amp if it's power supply was only 50% efficient!
But even on a smaller scale, the efficiency of a SMPS often makes a lot of sense. If the analog circuitry is properly designed then the noise from the power supply gets rejected by the analog circuitry and doesn't impact the audio noise (very much).
For those super-noise-sensitive applications you can do a hybrid approach. Let's say that you have an ADC that requires +5v. You can use a SMPS to generate +6v, then a super-low-noise linear regulator to bring that down to +5v. You get most of the benefit of the SMPS, but the low-noise of the linear regulator. It is not as efficient as just a SMPS, but those are the trade-offs.
But one thing to keep in mind... A SMPS for audio applications needs to be designed with audio in mind. Of course you'll need better filtering on the output. But you will also need to keep other details in mind. For example, at very low current the SMPS might go into something called "burst mode" or "discontinuous mode". Normally a SMPS will switch at a fixed frequency, but in one of these modes the switching will become somewhat erratic. That erratic behavior might push the output noise into the audio frequency band where it becomes more difficult to filter out. Even if the SMPS is normally switching at 1 MHz, when in one of these modes you could get 10 KHz noise. Controlling how this happens depends on the design of the chip that the power supply uses. In some cases, you can't control it. In that case you have no choice but to use a different chip or use a hybrid approach.
Some people advocate using only linear power supplies for audio. While linear supplies are less noisy, they have lots of other issues. Heat, efficiency, and weight being the biggest ones. In my opinion, most of the people who preach linear supplies only are either misinformed or lazy. Misinformed because they don't know how to handle switching supplies or lazy because they don't care to learn how to design robust circuits. I've designed enough audio gear with SMPS to prove that it can be done without too much pain.
A class D amplifier is a switching power supply. Those are more common these days and can have quite good specs. Audiophools may wrinkle their noses when they are told a amplifier is class D or has a switching power supply inside, but such a thing is harder to detect in a proper double blind test. In the audio world, it can be difficult to separate the science and measurable results from religious belief.
Short:
SMPS are much used in many audio systems.
In very top-end enthusiast targeted systems an iron cored transformer based supply may be preferred because of nuances in effect which are so fine that they can only be detected or claimed to be detected by true aficionados.
SMPS are regularly used to power audio circuits in many applications.
Most domestic audio equipment probably uses SMPS.
Top end systems for audiophiles may use "iron transformers" because of actual and or perceived benefits. Noise elimination for 50 Hz transformer based supplies is well understood, most of the noise energy is at low frequencies which are a multiple of the main frequency which makes it able to be rejected by notch filter techniques if astoundingly high levels of rejection are desired. The main exception is probably diode switching noise caused by current peaks when the diodes conduct at the peak of the AC waveform, and this can be greatly reduced by spreading resistors and generally good design.
SMPS typically use switching frequencies in the 50 kHz to about 2 MHz range and usually in the few hundred kiloHertz range. These SHOULD be even more readily filtered out that than low frequency noise, but rejection levels of amplifier circuitry decrease with increasing frequency and will often be far worse at above 100 kHz compared with at say 10 kHz.
Whether a well designed SMPS supply is any more liable to significantly affect the quality of a top end audio system is open to debate - and much debate has occurred on this subject. BUT if users THINK that SMPS MAY be worse than a traditional supply and/or if suppliers state that they are or may be, or that listening tests have confirmed that they are, then "modern stuff" is liable to be the loser when compared to iron cored supplies - regardless of what the reality may be.