Let's do some simple math(Corigé)
Let's say you record at 8bit mono at a frequency of 16 kiloHz. Cause 16 kiloHz, unless you a bat, is a normal hearing/speaking maximum(high pitch)
Well that mean that every second you need 8 bit × 16 kiloHz = 128 kilobitHz AND 128 kilobitHz × (1/1Sec) ÷ 1Hz = 128 kilobit/Sec or if you prefer 16 kilobyte/Sec.
On a Uno you have 32 kilobyte of flash(could do stéreo after all), and according to here: Flash memory - Wikipedia A normal writing cycle for flash is 150 kilobyte/Sec, well your buffering point fall apart.
Here Carte SD — Wikipédia they say that you can write at 150 kilobyte/Sec to 4 mégabyte/Sec to a SdCard, about 9,375 to 250 time faster then required.
So for a minute of recording you will need 128kilobit/Sec × 60 Sec/1 Minute = 7,68 mégabit/Minute , so a little less than 1 mégabyte/Minute.
On a SDcard of 4 gigabyte you have 4000 mégabyte. So 4000 mégabyte ÷ 1 mégabyte/1 Minute. That mean, you have pretty much access to 4000 minutes of low quality sound or if you prefer voice recording quality. Since the point of this tread was to make a voice recorder, well...
Finally, for all i know, the Uno is running at 16 mégaHz, so we are way way above the 16 kiloHz required to sample and record, no?
To conclude, the codes already exist and the numbers talk for them self. So since i'm not a arrogant man, i'm not gonna ask for apology. 
By the way i'm into physic so math is my hobby.