samtools sort doesn't support sam files now, you may use the following command to transfer SAM to BAM:

samtools view -b DICDvenus1_SortedReads.sam > DICDvenus1_SortedReads.bam

Then you could use DICDvenus1_SortedReads.bam to run "samtools sort"

I think this should work so long as the sam file has headers:

samtools view -Sb file.sam | samtools sort -n - file.sort

Not a samtools answer, but BBMap has a program which will do it, "sortbyname.sh". Usage:

sortbyname.sh in=DICDvenus1_STARAligned.out.sam out=DICDvenus1_SortedReads.sam

I'm sure there's a way to do it with samtools, but I don't know what it is offhand.

Edit: In light of Derek's answer, no, it's not possible, which makes the name "samtools" seem weird.

P.S. And incidentally, sortbyname supports sam, fasta, fastq, and also (if samtools is installed) bam files. It also has various other sorting modes.