If you use the most recent samtools version from the github repository you'll be able to set unit of memory and use threads, makes a massive difference!

https://github.com/samtools/samtools

$ samtools sort
Usage:   samtools sort [options] <in.bam> <out.prefix>
Options: -n        sort by read name
     -f        use <out.prefix> as full file name instead of prefix
     -o        final output to stdout
     -l INT    compression level, from 0 to 9 [-1]
     -@ INT    number of sorting and compression threads [1]
     -m INT    max memory per thread; suffix K/M/G recognized [768M]

Does the samtools sort maximum memory parameter (m- ) require / accept values given in bytes, kB, MB of GB?

sort answer: no

$grep  "max_mem" bam_sort.c 

size_t max_mem = 500000000;
    case 'm': max_mem = atol(optarg); break;

but you can always use a bash inline multiplication:

samtools sort -m $((10*1024)) ....