I opened up one of my Game Boy Advances (AGB-001) and took some measurements.
When measuring the potentiometer between pins 1 and 3 in-circuit, the initial resistance I measured was circa 7k Ohm, increasing rapidly to 10k+ and slowing, then tailing off around 18k. This is reasonable, given that there are three capacitors shown in parallel with the potentiometer (C50, C51, C59), which charge up at a non-linear rate when a multimeter is applied to the circuit. I'm mainly quoting these values as something that's easy for others to measure and compare against.
Having desoldered the contacts from the board, I measured the resistance between pins 1 and 3 as 26.5k. This would be slightly outside a 10% tolerance band for a 30k resistance (30k - 3k = 27k), but could be reasonable given its age.
I then proceeded to desolder one of the potentiometer's mounting points, and lever the package up to have a look underneath.
At first glance, all I saw was the mark 18C. There is, however, another mark, higher up, and fainter. It's slightly obscured by the scratches I made in levering the potentiometer off the board, but it appears to read 334. Confusingly, this would indicate 33 x 10^*4, or 330k, an order of magnitude higher than what I measured! (18C could be an EIA-96 code, but that would represent 15k.)
In short, neither of these possible values match the schematic, nor the value I measured on the aged component.
Package markings aside, this discrepancy in volume is unlikely to have much impact on the performance of the GBA's audio. Assuming the VOL pin on the AMP chip is a high-impedance input (drawing little current), applying circuit theory to the 30k potentiomenter turning from 0% to 100% would give a range of voltages from 0.401V to 2.959V, assuming the batteries were providing exactly 3V. Swapping this for a 26.5k pot gives a similar range of 0.445V to 2.955V. If a 10k part were used, it'd be 0.929V to 2.907V.
Based on these numbers, the exact resistance value of the potentiometer is not important in this circuit, and Nintendo may have chosen to use other convenient values in GBAs at other times during its production. Swapping in a 10k part results in a loss of ~0.58V range in voltages to the VOL input, or 22.7%. This is practically all at the bottom (quiet) end of the volume scale, so it'll have no practical effect on how loud the sound output can be.
In my personal experience, the bottom end of the volume range was too quiet to be audible on the built-in speaker, and I'd have to turn it up a good quarter of the way in order to start hearing anything. The loss of that part of the range would therefore be inconsequential.