Name: GWCtech AA1570 USB 7.1 Sound Card (aka Conrad AA 1570 7.1 SoundBox)

lsusb Output:

Bus 001 Device 004: ID 0d8c:0102 C-Media Electronics, Inc. CM106 Like Sound Device

Status:
Works out of the box – great!

2 Channel (Wave, 44.1kHz, 16Bit PCM, 1411kBit/s) and 6 Channel (Wave, 44.1kHz, 16Bit PCM, 4233 kBit/s) playback works fine. Hardware mixer works.

8 Channel (Wave, 44.1kHz, 16Bit PCM, 5644 KBit/s) stutters a lot. Sound is awful.
Simple signals (e. g. Sine Wave on all 8 Channel) are played correctly.
As soon as the signal gets more complex (8 Channel Music Track) it begins to stutter.

I know this is a lot of data that has to be processed.
I assume the bottleneck is the USB bus as the CPU load is around 5-10%.

Is there are a way to track the bottleneck?
My idea would be to play music directly from the SDCard to lower the overall USB traffic.
Is this possible? Ramplay is not an option as I have only a rPi with 256 MB RAM.