Well, from a hobbyists perspective, it would be great if the microcontroller and other components are easily sampleable.
ST has a great sampling program for their STM32F series of microcontroller, which you can get in up to 256k flash/64k RAM. Out of that I would go with either the STM32F105 or 107, because they have an I2S interface for sound. The STM32F107 series also comes with Ethernet. Sound could be done through a PCM1748 Stereo DAC (found it on ladyada's mintyMP3 page), which is also sampleable from TI. I would favor the 64-pin packages, mainly for soldering reasons, though they're available in 100-pin as well. One drawback though, is none of them seem to have external RAM interfaces, so it would have to be done in software. The only microcontrollers ST offers for sampling with that are the older STR7 series, which don't have nearly as many features, and ar ARM7TDMI based rather than the STM32F's Cortex-M3, and run slower. (If I remember correctly.) Also, the STM32F series is featured in some ARM-based Arduino lookalikes (and have the software to match) and are available from Futurlec in $25 dev board, though these are STM32F103s (No I2S for sound, but I'm sure it's still possible somehow). Also, you don't need a JTAG interface to program them, because they have a built in serial bootloader. You just need a USB-to-serial dongle and a python script I found using google. That alone makes the console at least $20 less (If you have a parallel port) to build (For some reason ARM-JTAG seems to be expensive, with a USB dongle being $50-70). Most hobbyists probably have a USB-to-Serial doohickey laying around if they've worked with any other microcontrollers (I think).
I've been taking a look at some of Atmel's offerings, and they look good, but I can't tell how their sampling works. TI offers a lot of Cortex-M3s for sampling, but I haven't had the time to attempt to sort through their system. It's really hard to figure out what MCU will do what on their site.
I couldn't figure out how to sample from NXP's site, looks like you might not be able to.
Disclaimer: I haven't actually done any ARM hacking myself- it just seems that there's a lot of schematics and stuff out there for the STM32F series, and they seem to be readily available.