Using a MCU's UART as DAC
While the fixed-value start and stop bits at the output of the UART will impose some minor limitations on what you can achieve, it would certainly be possible to use a software delta-sigma modulator to produce the rest of the data bits from the analog value that you want to convert. You could even write the modulator so that it accounts for the fixed bits and optimizes the rest of the data around them.

However, 400 kHz is still not a great sampling rate for this kind of application, and you'll be trading off resolution for bandwidth. For example, if you want "telephone grade" audio at an effective 8-kHz sample rate, that's only 50 output bits per sample, or about 5-6 bits of effective resolution.

If you want "CD-quality" audio, you'd need to be operating the UART at several MHz in order to get both the bandwidth and resolution you need. For example, an oversampling ratio of 256x and a sample rate of 44.1 kHz (typical values for audio-grade DACs) would work out to 11.2896 MHz.