The patches below (extracted from kernel version 2.6.24.3) 
do bulk->int change and let the kernel recognize low-speed 
midi devices, the old 2.6.22.1-rt9 kernel said: 
"unknown device speed".
If you're using a new kernel >=2.6.23 there is no need for 
this patch. But I use the older 2.6.22.1-rt9 kernel because 
it works much better with my laptop and jack, with newer ones 
I get xruns constantly.
The patch consists of two parts: 
one for /usr/src/linux/drivers/usb/core/config.c:

--- config.c   2007-07-10 20:56:30.000000000 +0200
+++ config.c.2.6.24.3   2008-01-24 23:58:37.000000000 +0100
@@ -124,6 +130,21 @@
       endpoint->desc.bInterval = n;
    }
 
+   /* Some buggy low-speed devices have Bulk endpoints, which is
+    * explicitly forbidden by the USB spec.  In an attempt to make
+    * them usable, we will try treating them as Interrupt endpoints.
+    */
+   if (to_usb_device(ddev)->speed == USB_SPEED_LOW &&
+         usb_endpoint_xfer_bulk(d)) {
+      dev_warn(ddev, "config %d interface %d altsetting %d "
+          "endpoint 0x%X is Bulk; changing to Interrupt\n",
+          cfgno, inum, asnum, d->bEndpointAddress);
+      endpoint->desc.bmAttributes = USB_ENDPOINT_XFER_INT;
+      endpoint->desc.bInterval = 1;
+      if (le16_to_cpu(endpoint->desc.wMaxPacketSize) > 8)
+         endpoint->desc.wMaxPacketSize = cpu_to_le16(8);
+   }
+
    /* Skip over any Class Specific or Vendor Specific descriptors;
     * find the next endpoint or interface descriptor */
    endpoint->extra = buffer;

the second one for /usr/src/linux/sound/usb/usbaudio.c and .../usbmidi.c:
Code:

--- usbaudio.c   2007-07-10 20:56:30.000000000 +0200
+++ usbaudio.c.2.6.24.3   2008-01-24 23:58:37.000000000 +0100
@@ -3381,7 +3390,8 @@
 
    *rchip = NULL;
 
-   if (snd_usb_get_speed(dev) != USB_SPEED_FULL &&
+   if (snd_usb_get_speed(dev) != USB_SPEED_LOW &&
+       snd_usb_get_speed(dev) != USB_SPEED_FULL &&
        snd_usb_get_speed(dev) != USB_SPEED_HIGH) {
       snd_printk(KERN_ERR "unknown device speed %d\n", snd_usb_get_speed(dev));
       return -ENXIO;
@@ -3455,7 +3465,9 @@
       usb_make_path(dev, card->longname + len, sizeof(card->longname) - len);
 
    strlcat(card->longname,
-      snd_usb_get_speed(dev) == USB_SPEED_FULL ? ", full speed" : ", high speed",
+      snd_usb_get_speed(dev) == USB_SPEED_LOW ? ", low speed" :
+      snd_usb_get_speed(dev) == USB_SPEED_FULL ? ", full speed" :
+      ", high speed",
       sizeof(card->longname));
 
    snd_usb_audio_create_proc(chip);


--- linux-2.6.22.1-rt9/sound/usb/usbmidi.c   2007-07-10 20:56:30.000000000 +0200
+++ linux-2.6.24.3-rt3/sound/usb/usbmidi.c   2008-01-24 23:58:37.000000000 +0100
@@ -963,8 +983,10 @@
       snd_usbmidi_out_endpoint_delete(ep);
       return -ENOMEM;
    }
-   /* we never use interrupt output pipes */
-   pipe = usb_sndbulkpipe(umidi->chip->dev, ep_info->out_ep);
+   if (ep_info->out_interval)
+      pipe = usb_sndintpipe(umidi->chip->dev, ep_info->out_ep);
+   else
+      pipe = usb_sndbulkpipe(umidi->chip->dev, ep_info->out_ep);
    if (umidi->chip->usb_id == USB_ID(0x0a92, 0x1020)) /* ESI M4U */
       /* FIXME: we need more URBs to get reasonable bandwidth here: */
       ep->max_transfer = 4;
@@ -976,8 +998,14 @@
       snd_usbmidi_out_endpoint_delete(ep);
       return -ENOMEM;
    }
-   usb_fill_bulk_urb(ep->urb, umidi->chip->dev, pipe, buffer,
-           ep->max_transfer, snd_usbmidi_out_urb_complete, ep);
+   if (ep_info->out_interval)
+      usb_fill_int_urb(ep->urb, umidi->chip->dev, pipe, buffer,
+             ep->max_transfer, snd_usbmidi_out_urb_complete,
+             ep, ep_info->out_interval);
+   else
+      usb_fill_bulk_urb(ep->urb, umidi->chip->dev,
+              pipe, buffer, ep->max_transfer,
+              snd_usbmidi_out_urb_complete, ep);
    ep->urb->transfer_flags = URB_NO_TRANSFER_DMA_MAP;
 
    spin_lock_init(&ep->buffer_lock);
@@ -1323,6 +1351,13 @@
          endpoints[epidx].out_ep = ep->bEndpointAddress & USB_ENDPOINT_NUMBER_MASK;
          if ((ep->bmAttributes & USB_ENDPOINT_XFERTYPE_MASK) == USB_ENDPOINT_XFER_INT)
             endpoints[epidx].out_interval = ep->bInterval;
+         else if (snd_usb_get_speed(umidi->chip->dev) == USB_SPEED_LOW)
+            /*
+             * Low speed bulk transfers don't exist, so
+             * force interrupt transfers for devices like
+             * ESI MIDI Mate that try to use them anyway.
+             */
+            endpoints[epidx].out_interval = 1;
          endpoints[epidx].out_cables = (1 << ms_ep->bNumEmbMIDIJack) - 1;
          snd_printdd(KERN_INFO "EP %02X: %d jack(s)\n",
                 ep->bEndpointAddress, ms_ep->bNumEmbMIDIJack);
@@ -1336,6 +1371,8 @@
          endpoints[epidx].in_ep = ep->bEndpointAddress & USB_ENDPOINT_NUMBER_MASK;
          if ((ep->bmAttributes & USB_ENDPOINT_XFERTYPE_MASK) == USB_ENDPOINT_XFER_INT)
             endpoints[epidx].in_interval = ep->bInterval;
+         else if (snd_usb_get_speed(umidi->chip->dev) == USB_SPEED_LOW)
+            endpoints[epidx].in_interval = 1;
          endpoints[epidx].in_cables = (1 << ms_ep->bNumEmbMIDIJack) - 1;
          snd_printdd(KERN_INFO "EP %02X: %d jack(s)\n",
                 ep->bEndpointAddress, ms_ep->bNumEmbMIDIJack);